There are a lot of reasons for this:
- The first reason of this lies in Palkia's inferior type coverage in Ubers. Water and Dragon STAB are commonly blocked in Ubers—Primal Groudon the most common Pokemon in Ubers is immune to Water STAB. On the other hand, popular fairy-type Pokemon like Arceus-Fairy and Xerneas are immune to STAB Spacial Rend. All of this makes Palkia heavily prediction reliant.
- Arceus-Fairy is especially annoying—it often carries enough 96 EVs in Spe to hit up more than of Palkia's maximum speed. It is the most reliable checks, it can pretty much easily Recover stall Palkia.
- Z Heal Block is good with Sticky Webs I guess, but there are better Z-Move users than Palkia.
- Basically, Palkia requires too much prediction and doesn't yield enough awards to justify it's usage a lot in Gen 7 Ubers.
- Another one is what the Smogon analysis covers:
Palkia can use Psychium Z in conjunction with Heal Block to grant it a one-time +2 boost to its Special Attack. While this can be devastating against certain defensive cores, Palkia will lose the boosts if forced out, which makes the consistent power of Life Orb a better choice most of the time.
What I told is also mentioned in the analysis, as well.
Because both of its STAB moves are blocked by very threatening Pokemon such as Primal Groudon and Xerneas, Palkia can be a hit-or-miss Pokemon in battle, as it depends very much on its attacks finding the correct mark.
Palkia's lack of Speed-boosting options makes it extremely limited against offensive teams.
Arceus-Fairy: Arceus-Fairy is the best general answer to Palkia, boasting an immunity to Dragon-type attacks and the bulk to shrug off Palkia's other moves with Recover. It naturally outspeeds Palkia and can swiftly KO with Judgment.
Source (along with a bit of help from a helpful Ubers player named The Contrast.)
Hope this helps!