0. JS world is full of people captivated by shiny things.

1. yarn was a shiny version of npm

2. They did a Python 3 stunt (well...much worse) with yarn 2 so nobody uses it and everyone stuck with 1.x

3. Bugs aren't fixed in 1.x

4. So back to npm, or the next shiny thing.

Edit: no you can't switch back and forth. The tools end up storing internal state in your project repo. I mean, yes you can change, but it'll mean some hassle and possibly re-writing build files.