Probably because mongoose is so well written, it feels inferior to use another kind of ORM library. I'm guessing nobody has written something as good as the activerecord library in the ruby world for node.
I mean you're kind of trying to say "Why not X?" The answer is X would probably work fine, but Mongoose got there first, set the standard, has lots and mindshare, and is very well written and tested.
Could most everyone use D instead of Go or Java? Yeah, probably they could. Is one significantly objectively better than the others? Probably not.
I've used Mongoose extensively and it just works. It feels really idiomatic and has a lot of love in the community. I don't have a reason to look anywhere else.
