It's an entrenched network effect. There's a lot of existing scripts written targeting POSIX shells, and there's an incentive to have all your distro maintenance scripts written in one language if possible.
If you want to switch your existing distro scripts to fish, you need to rewrite a lot of stuff. If you want to start a new distro, "it's written in fish!" isn't a terribly compelling selling point, since I don't think anyone is actively picking distros based on their tooling language.
For the general devops case, there's a lot of existing example code out there for bash scripts, and far less for fish. "How do I do X in bash?" is probably going to get you a decent example on Stack Overflow. Devops (I feel) cares a bit more about the installed-everywhere thing, and fish isn't a standard part of most distros.
Oil having a goal of being completely sh/bash compatible gives it much greater odds of something like Debian switching to it, since it wouldn't carry huge technical debt along with it.