Backwards-incompatible change breaks code. And it may do so silently. And the C standardisers don't really have the political clout to do this. Trying to force people to "new C" would be a lot of work and would probably result in a culture split like the backwards-incompatible Perl and Python splits. OpenBSD have managed to move people to strlcpy(), but that's about it.

People are just as likely to migrate to another language (Rust? Go? Even C++) as upgrade the old C code.