Bats isn't a port of anything, and sometimes Bash is the right tool for the job.

I maintain two other projects written in Bash (rbenv and ruby-build). I looked, and couldn't find any suitable testing tools for Bash, so I created Bats.

My criteria for deciding what to work on in the open-source world is: do I need something that doesn't yet exist, or something whose existing implementations are unsuitable? If yes, I try to fill the void as best I can, and package it up nicely so others can use it too.