Because it's an editor, and if it just quit on me every time I mistyped Ctrl-C just because you can't be bothered to type the right command, I would be quite angry.
Since I switch between Mac OS X, Windows, and Linux hosts, accidentally typing ctrl-c when I meant to type cmd-c (copy outside of Terminal) isn't such an infrequent error. I'm thankful that ctrl-c isn't mapped to quit without saving.
Well, and my GUI editors don't respond to single-letter commands. It would violate my expectations of their UIs if they did. Similarly, in vim, I expect ctrl+q to send an XON signal to my terminal emulator...not to quit my editor.
They're different programs with different UI conventions. I don't expect them to act the same. And I especially don't expect vim to change, after having 25 years of its own precedent and an additional 16 through vi.