There are at least two, though they may not be relevant for everyone: POSIX compatibility and ubiquity on random systems where your ability to install things is limited.

Some things won't work out of the box if your shell isn't posix compliant. For example, I once installed Ocaml, and its installer assumed my shell was posix, so some of the commands broke (I forgot the exact details now). Also NVM (node version manager) doesn't work with fish (or at least at the time).

Sometimes there are alternatives or workarounds, but the fact is that using fish does come with that downside. Doesn't come up often, but it is annoying when it does come up.

I like fish btw, just saying that I disagree with your "no reason to continue using zsh" comment. I use zsh because I was fed up with random things breaking due to not having a posix shell. And zsh can be customized to have most of the niceties of fish anyway. But it is pretty cool how fish comes with so many niceties out of the box!

Well, I like it and that’s a good reason for me.