Why not?
But okay, most other Rain support pokes don't do as well against Toxicroak, but you have to explain why this is important in the context of what is broken, and why it is only Uxie that breaks anything.
You might have a point, but so far you haven't even tried to make a valid argument that justifies it. And any argument based on only what you personally have trouble with is not a good one at all.
Counters to standard Rain pokes:
Toxicroak
Paralysis
Scarfed Electrode with Thunder (lol)
Tracer with T-bolt/Thunder (Garde/Pory2 mainly)
Golduck
Other than those, rain pretty much shits all over everything.
Uxie hard counters Toxicroak, outspeeding and OHKOing with Psychic or Zen Headbutt (or HP Psychic if you're a moron) and only loses to a +2 Dark Pulse/Shadow Ball after SR which is not going down thanks to Electrode lead.
Uxie eliminates all paralysis with Heal Bell, rendering sacrifice paralysis on Kabutops/Gorebyss useless.
Scarfed Thunder Electrode is completely nonviable outside of this one situation.
Uxie straight up walls Gardevoir and Pory2 and doesn't mind any status they send your way.
All Golduck can do is Encore Heal Bell or Rain Dance, which doesn't really make too much of a difference to Uxie.
Uxie is one of the best walls in UU, it is essentially a lesser Cresselia, boasting similar defensive stats and a better movepool. He is by far the sturdiest pokemon that can set up Rain Dance and isn't fazed by anything short of a boosted SE Attack (which won't happen if the Rain sweepers are doing their job.) He also has base 95 speed, meaning even if he doesn't invest in speed, he still outspeeds other walls and slow sweepers.
Combining the durability of Uxie with the enormous offensive power of Rain Dance is a problem. Uxie personally nullifies the majority of RD counters in addition to being an excellent setup poke.