The main thing that really has dropped mences viability to the depths of obscurity was the introduction of fairy types and the whole slew of Mega Dragon Dancers XY+ORAS introduced. Mence lost the ability to spam Outrage and gather up boosts with moxie because of the sheer popularity of fairy types such as Azumarill, Clefable and Altaria, which are all on basically every team in combined usage. While immunities aren't really a problem for alot of mons, Fire types do fine with Heatran around and ground types are still amazing even with all these levitators and Flying types around, the problem Mence faces with these mons is the free turns it hands out to these very dangerous set up sweepers. Mega Altaria is extremely hard for offensive teams to handle once it gets to +1 with its coverage and power, Clefable is very east to snowball with because of the ease it can continuously set up calm minds with, and Belly-Drum Azumarill is nigh-impossible for weakened offensive teams to deal with. This is especially annoying for mence because it does best on hyper offensive teams, which really struggle with set up mons outside of prankster T-Wave and weak priority, which can often not be enough for threats such as BD Azumarill and DD + Refresh M-Altaria. this often makes Mence a liability to run on most teams, and is one of the reasons its not used very often.
With the whole slew of Mega Dragon Dancers XY brought us, such as Altaria, Zard-x, Tyranitar and Gyarados, as well as other mega set up sweepers such as Houndoom, Pinsir, Gallade, Heracross and scizor. Now, while you may be perfrectly correct in thinking "But Teeny! opportunity cost is a thing which restricts these mons, right?", Dragon Dancers and set up sweepers in general are pokemon that thrive with teams built around them, and seeing as you'll often be picking this mon first, you would be inclined to make the obvious choice in picking the much more dangerous mega pokemon instead of the puny salamence. Now yes, it is often advised against building around mega pokemon, for good reason, these pokemon are so good at what they do that they are worth the mega slot they take up, and often your team will want the remaining 5 slots to support, wallbreak, and work with the sweeper, and salamence finds itself having nowhere to fit in, and is passed up so often on common OU teams.
Metagross, Scizor, Gengar, and Venusaur all handle fairies very well, being able to come in on all of the ones you mentioned and OHKO most of the time. And since your mega slot isn't being used by Salamence they make great teammates for him, something no other mega can say. People seem to be stuck on just Spamming a Dragon move with Mence to get boosts, instead of say, using fire blast on scizor to grab a boost, or earthquake on Heatran, two pokemon that are often used as dragon checks in OU. Mence also doesn't provide free set-up unless the person using it is literally spamming outrage. Here are some Calcs:
252 Atk Life Orb Salamence Earthquake vs. 252 HP / 160 Def Clefable: 155-183 (39.3 - 46.4%)
252 Atk Life Orb Salamence Iron Head vs. 252 HP / 160 Def Clefable: 250-294 (63.4 - 74.6%)
252 Atk Life Orb Salamence Iron Head vs. 88 HP / 4 Def Azumarill: 139-165 (38.2 - 45.4%)
252 Atk Life Orb Salamence Iron Head vs. 92 HP / 0 Def Azumarill: 140-166 (38.4 - 45.6%)
252 Atk Life Orb Salamence Earthquake vs. 92 HP / 0 Def Azumarill: 177-208 (48.6 - 57.1%)
252 Atk Life Orb Salamence Earthquake vs. 88 HP / 4 Def Azumarill: 175-207 (48.2 - 57%) -- guaranteed 2HKO after Stealth Rock
252 Atk Life Orb Salamence Earthquake vs. 252 HP / 252+ Def Mega Altaria: 99-117 (27.9 - 33%)
So Most of the time he won't OHKO a switchin, but if we look at some mons that might be run with him:
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 252 HP / 252+ Def Mega Altaria: 218-260 (61.5 - 73.4%)
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 92 HP / 0 Def Azumarill: 195-229 (53.5 - 62.9%)
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 88 HP / 4 Def Azumarill: 195-229 (53.7 - 63%)
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 252 HP / 160 Def Clefable: 344-408 (87.3 - 103.5%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 160 Def Clefable: 452-534 (114.7 - 135.5%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Clefable: 372-440 (94.4 - 111.6%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 64 HP / 0 Def Mega Altaria: 392-464 (127.6 - 151.1%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Mega Altaria: 288-342 (81.3 - 96.6%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 88 HP / 4 Def Azumarill: 255-301 (70.2 - 82.9%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 92 HP / 0 Def Azumarill: 256-303 (70.3 - 83.2%)
0 SpA Mega Venusaur Sludge Bomb vs. 92 HP / 0 SpD Azumarill: 278-330 (76.3 - 90.6%)
0 SpA Mega Venusaur Sludge Bomb vs. 88 HP / 0 SpD Azumarill: 278-330 (76.5 - 90.9%)
0 SpA Mega Venusaur Sludge Bomb vs. 252 HP / 0 SpD Mega Altaria: 222-264 (62.7 - 74.5%)
0 SpA Mega Venusaur Sludge Bomb vs. 64 HP / 0 SpD Mega Altaria: 222-264 (72.3 - 85.9%)
0 SpA Mega Venusaur Sludge Bomb vs. 252 HP / 96+ SpD Clefable: 206-246 (52.2 - 62.4%)
0 SpA Mega Venusaur Sludge Bomb vs. 252 HP / 4 SpD Clefable: 252-296 (63.9 - 75.1%)
And its not like you are going out of your way to build around Salamence here, M-Metagross is S-Rank, Venusaur is A-Rank, and Scizor is A-Rank. And all of this is simply based off of having a DDMence.
A big Non-Mega Dragon Dancer that is also very commonly chosen over mence is the mighty Dragonite. Lets compare these two pokemons stats first up:
Salamence: 95 / 135 / 80/ 110 / 80 / 100
Dragonite: 91/ 134 / 95 / 100 / 100 / 80
So basically they have identical Attack stats (mence reaches 405 and D-Nite reaches 403), near Identical Special Attack stats (which is irrelivant regardless), and Mence has the edge in speed, which doenst really do muhc as D-Nite outspeeds all common OU pokemon at +1 except M-Sceptile, Beedrill . The Bulk is where D-Nite wins over however, mence slightly wins on Hp by 4 base stats, but D-Nite has a whopping 15 more Defence and 20 more special Defense, meaning it can set up a DD much easier and can take lots of priority. D-Nite also beats Mence in abilitys, Multiscale is like the holy grail of abilitys for a set up sweeper, which is saying something considering how good Intimidate is on set up sweepers. Multiscale basically allows D-Nite a guaranteed set up against all mons except Weavile and mamoswine, and it can then wreck havoc on the opposing team with its powerful outrages and great coverage.
Now, the final thing that really restricts Mences viability is its lack of a secondary STAB to hit Dragon resists with, or the lack of any secondary STAB at all tbh. Zard-X has Fire to form a nearly unresisted STAB combo with dragon, Gyara has Water + Dark, which is very good offensively with few resists in the OU tier and M-TTar has a fantastic Dual STAB combo that hits a large majority of the tier super effectively. This leaves Mence to rely on both Earthquake and Iron tail to hit fairy and steel types, and it can't really give either of these up for fear of being walled by common mons. This means it doesn't have the option to run moves such as roost, sub, tailwind etc. in its moves, which gives it a slight case of 4MSS, and leaves it easily walled by pokemon such as Azumarill and Scizor that resist/are immune to its dragon STAB and easily take the neutral non-STAB coverage moves that Mence tries to throw at them.
So really the only niche I see Mence having is as a lure/late game cleaner on dragon spam teams, I wont post a set because its already been stated above.
TL;DR
Mence was left behind with the many new editions of XY, and finds itself struggling to find a proper placement in the meta, but has a very small niche on dragon-spam teams
Personally I prefer Mence's speed over DNite's. Being outrun by a couple of Scarfers in the tier even after a boost sucks, particularly Kyurem-B, who OHKO's Dnite. Dnite has extremespeed to cover for that though. Really this is the only argument that I have seen that makes any sense, and yet it doesn't. They are different enough that both fill different slots on teams. Also from personal experience Dnite's ability only comes up in half of the battles he is in, because SR. You can say you run defog support but we all know that that doesn't guarantee the hazards will be gone when you need to switch-in. Intimidate may be slightly worse but at least it works all the time.
I give you the STAB thing, but Dnite suffers from the same and still sees usage.
Again I feel Salamence can shine in OU. It does suffer from competition as a dragon, but it has more then enough to set it apart from others and is completely capable of performing in OU. Also as a side note I had a team with Salamence on it that went 24 straight wins and no losses, so I know he can perform in OU.
A bit rushed here at the end, have to head to class, so I apologize if anything doesn't make sense. Will be back though.