Because people were triggering it by accident all the time. Firefox and Safari don't navigate back on backspace either.
Enter only auto-submits if there's a visible submit button or if there's just one input, and that behavior goes back as far as I can remember.
It should have been Ctrl-Backspace or something.
About enter to auto submit, i've been running into this problem more often where some web frameworks just ignore the Enter key. There is a submit button, but i cannot trigger it via the enter key.
It's also far less likely to be accidentally typed while trying to fill in a form and fumbling the tab navigation.
Because alt+left is a better choice? Really f**ing annoying when some dumb javascript removes focus from a text box you're editing and the page navigates back.
I can kinda get that one.
Because web pages don't preserve element focus well, web was originally meant for documents in the first place, not UIs. If you're trying to erase a character and the focus has been lost, you go back in history instead. It's a real existing problem in poorly designed pages, not a nefarious plot to annoy power users by Google, Microsoft, or Mozilla. It was always janky as hell.