Bash has had this for ages too. Debian comes with it in the "bash-completion" package, for example. People get all excited about trivial enhancements for Bash for the same reasons they don't know about Zsh: The pain isn't great enough for them to actively seek out alternatives.
So why aren't they on by default? Why is the latest Ubunbtu (styled as a user friendly Linux not a purist Linux) still making the clueless "HUH?" noise at me during tab completion because it doesn't have completion-ignore-case on by default? Or automatically showing a list of clashes or cycling through clashes instead of sitting there like a lemon? Both also one line of config somewhere.
Because not enough people care enough to push hard enough for it to change - presumably most of the people who do care don't find having to do apt-get install bash-completion onerous enough to matter.
