So many reasons all of them good.
In BW2 Palkia's primary function was to check Kyogre. In ORAS, that role has become overshadowed by Primal Groudon which takes 0% from Water moves upon switch-in as well as from Thunder. Since Lati@s is hit by Ice Beam super effectively with Desolate Land in one fell swoop Primal Groudon rose to become the #1 check to Primal Kyogre in the tier and Primal Kyogre itself's viability and thereby popularity decreased. On a side note, the argument that "Palkia no longer checks Primal Kyogre" is bullshit, like how does it not when it can throw Specs Thunder and Dragon Tail on a non-choice set if it tries to set up Calm Mind on your face. xD In the end though it's more Primal Groudon's omnipresence that unjustifies Palkia's use on any Ubers team right now.
Regarding Palkia's offensive qualities, first, the vanishing of the omnipresent Kyogre means that Hydro Pump is no longer boosted by rain. Its other STAB has been met with the introduction of a new type which is immune to it, while said type also hits Palkia itself super effectively. The stuff it used to threaten with its coverage moves like Fire Blast and Thunder have lost their place in Ubers, too, like many Steels whom relied on the rain to provide a cushion for weaker Fire attacks while, again, Kyogre is gone so Thunder is a lot less useful.
The return of Soul Dew in ORAS from XY didn't help its viability relevant to the metagame, as its one-dimensionality compared to the Lati twins choked its fitting on various archetypes. Also let's be honest the Rest set some people used in XY was utter shit lol sorry not sorry.
Anyway as a BW2 player my honest feelings about this Pokémon are as follows: in BW2 it was a good Kyogre counter that was offensive, unlike the plethora of other Kyogre counters in the tier. It was arguably better in DPP since the Lati twins were shakier as Kyogre check since they had the potential to get Pursuit-trapped. If you wanted to not get 6-0ed by Scarf/Specsogre on an offensively-inclined team, you had three options: this thing or the Lati twins. While different coverage, Calm Mind, reliable recovery, and Healing Wish made the latter two alluring, Palkia differentiated itself by (1) being more solid as a Kyogre counter (it wasn't hit by Ice Beam super effectively), (2) having a strong water move under rain which was on often, and (3) having access to a strong Fire move. Under rain its Hydro Pump gave you like a secondary Kyogre almost, under sun suddenly its strongest move became Fire Blast, and on sand teams Leftovers recovery was possible. The reason Palkia is so bad now is because all of the aforementioned three points have been rendered invalid as a result of (1) the introduction of Primal Groudon, (2) decrease in Drizzle's omnipresence, and (3) decrease in relevancy of said Fire moves as the lack of rain led to a decrease in the use of Steels Fire Blast itself was meant to target like Ferrothorn, respectively.
A good way to understand this concept is by just asking yourself this question honestly: what does Palkia do in this metagame? If you can actually justify its use then consider yourself a genius! You've proven yourself to be more creative than the entire ORAS Ubers community thus far.