There can be a ton of reasons, albeit I personally also just stick with default (for me zsh). In typical linux user fashion I also must tell you that bash and zsh are shells, not terminals.

The two main reasons you’d choose a particular shell is because you prefer it’s configurability or syntax. Zsh has a bunch of features that you can enable and you can configure it to behave basically however you want, like adding spelling correction or multiline editing, but it’s defaults absolutely suck unless your distro comes with a sensible config. Fish, which another guy here’s raved about, goes in basically the opposite direction and is really nice to use out of the box (I haven’t used it though). I hear it’s technically not a valid /bin/sh substitute like zsh or bash because of syntactic differences, but that’d be a whole other rabbit hole if true.

One other reason can be performance concerns because bash is pretty slow when treated as a programming language, but I’d argue you shouldn’t organize your workflow so that bash is a performance bottleneck.