Because other languages available at that time lacked certain programming conveniences found in bash :) Despite its shortcomings, it’s still a very convenient language for running other programs, working with files, and piping output from one program to another, or to a file. Bash was first released in 1989, and I don’t know exactly when it passed the threshold for widespread adoption, but I can’t think of anything that would’ve been a better alternative.
I think OP is also asking “why aren’t people switching to something else now” which is a completely different question.