sh shell is specified in POSIX, and it's 'always' there, but it's simply not as usable as bash. It's easier to start as a sh script and give into make it a bash script than to switch to something else later.

Perl has a reputation, and isn't always available. Python isn't always available either and has the 2 vs 3 problem still (maybe not for much longer).

Many scripts are super simple, or at least start that way, so something that starts as just a list of commands to run is a better interface; but it gets unweildy if you need deeper logic and nesting of things. Such is life.