Probably because having the default be something other than what a user is used to is not going to compel them towards doing the research to fix it.

In order to get cua-mode working in emacs, you first have to know that it exists. I didn't know it existed for like two years of daily emacs usage, I just can't find it in myself to expect someone to know about it after 10 minutes.

By the time you know it exists, you've already accepted the keybindings and you've already put in the time to build some muscle memory, and it doesn't seem as important anymore.

The fact that some people get past it isn't really an indicator that it isn't a huge friction point.