From a UX standpoint there's a big advantage to monoculture: the user learns a thing once and applies the idiom everywhere. For most users, diversity of options means having to learn every option. They'd rather have one good thing -- or even one good-enough thing.

The world does have a variety of options, so users need to live with the fact that they're going to have to discover things, and designers are going to have to live with the hassle of designing for users with a variety of experiences. That makes the best possible UX worse than it would otherwise be. It's a necessary evil, but it's not surprising that a lot of developers try to wish it away, especially when they can get away with that, at least initially.