I use fish for simple command pipelines as well. But traditional shells are not as good when I need to do anything “structured”, because they treats almost any value as a string and don’t have anonymous functions. The first problem means that you have to parse a string again and again to do anything useful, the second means that when both pipe and xargs fails you are doomed.
Nu solves both of the big problems that matters when you want to do rather complex but ad-hoc processing of data. And with a rather principled design, nu is very easy to learn (fish is already way better than something POSIX like bash though).
Personally another important reason is that I have a Windows machine at work and nushell is way easier than pwsh.