Tried another shell and liked it so much that you want to use it all the time by making it the default shell? Here's how you can change the default shell in Linux.
I don't use anything other than BASH, but I hear that ZSH has command tab-completion (not just directory tab-autocomplete) similar to PowerShell and also some other cool shit I'm not really bothered to check out since bash is the GOAT.
Nothing wrong with it, it's just boring. Fish shell has some new features that make it nice to use. So does zsh. Tab completions, history navigation, plugins and such.
Because you're bored. It says so right in the title.
Personally I don't see the point of changing the shell either. Bash is more than good enough for my use and any other shell is going to have the disadvantage of not being the ubiquitous standard so it is always going to have an uphill battle to dislodge bash.
That said, if people want to play around with a new shell just for the sake of it, why not? I like to play around with exotic window managers myself, not because my regular plasma desktop doesn't suffice, but because I like to try something different every now and then.
Fish is actually user friendly and easy to learn. The interactive completions are better than any other shell and are something I don't want to live without.
It differs from bash in some esoteric ways, but any issues you might encounter as a result are easily worked around by putting shebangs in your scrips, which you should be doing anyway, and bash -c 'your command'.
Honorable mention for nushell, but that one differs from bash a little too much to pick up quickly. However, having an object-oriented shell is pretty sick.