Politics mainly. It's already hard to convince people to give up part of their wealth for their fellow citizens. Good luck convincing them to give it up for non-citizens.

Also, logistics. If immigrants can get the UBI, then people would flood in from all over the world just to get UBI. You'd either have a ton of expense, or you'd have to make legal immigration nearly impossible.

It would certainly be challenging to be a non-citizen resident if you have to pay the same taxes as everyone else, but are cut out of such a major benefit.

Basically a gigantic extra tax for not having citizenship.

Without the right to vote, I think it would be hard to get much political will for it. Even if we started out giving it to noncitizens, it would be the first thing cut in budget cuts.