Sometimes you write POSIX shell (or very nearly POSIX) for portability. Sometimes you write in "a proper programming language" for more powerful/flexible features. And sometimes you write Bash to get both.

Bash is ported to basically every system, it's a small binary (by today's grotesque standards), and it gives you some added functionality that would be otherwise annoying to re-implement in POSIX sh (or Bourne sh). But it's also far less complex than Python. In today's world, if someone has any shell, it's a safe bet that they either already have Bash, or they can install it just like they'd have to install Python.

It's less painful in the long term to use Bash rather than Python. Python is more time-consuming and costly. It's a larger space requirement, you have to manage more runtime dependencies and a virtual environment, debugging is worse, and there's more opportunity for bugs because it's more complex. Bash scripts also rarely get bugs down the line due to dependencies changing, whereas this happens in Python so frequently we have to pin versions in a virtualenv. When's the last time you pinned the version of a Bash script dependency?