I get it #ifndef is cool and all, but why won't people start using #pragma once is beyond me...

#pragma once is not part of the spec and is not guaranteed to be supported by all compilers. In certain lines of work this can be an important detail.

That actually explains it a lot!

To expand on this a little; it's important in places like the games industry, where you'll likely have a common codebase running on multiple console platforms. If any single one of your target compilers doesn't support #pragma once, then your codebase has to use the standards-compliant #ifndef :)