I think reputation is a big one. Personally, I don't know a lot about European zoos, but I know that some highly regarded ones are Chester, Burgers, Schonnbrunn, Zurich, and the two Berlin Zoos. I'd be more inclined to put those collections higher up on a bucket list due to how well-regarded they are on here. That being said, crossing the Atlantic isn't really feasible for me right now so I don't have any European zoos on my bucket list.This is no criticism, but I am always surprised which European zoos feature and don't feature on bucket lists from non-Europeans. Makes you wonder which other factors play a bigger role than quality.
I'd imagine for other people, some factors could include the presence of species they can't see closer to home, notable exhibits, and the zoo's historical significance. To reverse this, however, sometimes I think the same thing about American zoos and find it interesting which ones make the bucket lists of non-Americans.