I'm going to be mentioning why Uxie fell from PU to ZU. It's because of these two mons outclassing Uxie in PU:
Xatu and Mesprit
Xatu
In Gen 8 it is outclassed in the lower tiers by Xatu which has Recovery in Roost and has Teleport to safety bring in teammates. It also has a superior Ability in Magic Bounce.
commented May 6 by J™
What J said is true. Uxie's only reliable source of recovery is Leftovers, and cannot recover if Uxie's Leftovers gets removed by Knock Off or is running Colbur Berry. Xatu has Roost to recover HP, and sure, Uxie gets U-Turn to bring in teammates, but Xatu having Teleport can reliably bring in teammates due to Teleport having negative Teleport priority. Uxie has no way of stopping hazards from getting on your side, whereas Xatu's Magic Bounce prevents hazards from getting on your side, barring Mold Breaker Stealth Rock Druddigon.
I guess I'll go ahead and mention this:
Xatu could move from PU to NU
Source
Due to Xatu's usage in being a good pivot and dual screens setter in NU, it may rise from PU in July, meaning Uxie can compete with one less psychic type. But in the meantime... it has to face competition with Xatu.
Mesprit
Here are Uxie and Mesprit's base stats:
Uxie's Base Stats:
HP: 75
Attack: 75
Defense: 130
Sp. Atk: 75
Sp. Def: 130
Speed: 95
Mesprit's Base Stats:
HP: 80
Attack: 105
Defense: 105
Sp. Atk: 105
Sp. Def: 105
Speed: 80
Even though Mesprit has defense stats and a speed stat that's lower than Uxie, it has a higher base special attack, meaning it can be more offensive while still being able to do the things a defensive Uxie does. Mesprit is also a Pokemon who can run different sets, depending on what the team Mesprit is on needs.
If we were to look at these three in the viabilty rankings, you'll find Mesprit at A rank, Xatu in A-, and Uxie in B rank. You can confirm this by looking in the link below:
PU Viability Rankings
TL;DR
In summary, Uxie fell from PU to ZU in Gen 8 since it faces competition from Xatu, who has reliable recovery, the ability to bring in teammates saftley, and a way of stopping hazards from going on your side with Magic Bounce. It is also overshadowed by Mesprit, who has more special attack, meaning it can be more offensive while still being able to do the things defensive Uxie does.
Hope this helps.
