Some good answers in here already. It boils down to a couple points for me:
Back when I started selfhosting, it was either nginx or apache, and I found nginx better and easier to set up
All the nginx knowledge I learned years ago still works just the same as it did back then, so why potentially mess things up by switching if it all still works
Basically every project has an example nginx config for reference, that can't be said about other proxies
It is easier to find support online for edge cases that might pop up with nginx due to the ubiquity of its use and years of history