Well, let’s be real: many C programs don’t want to rely on Glib, and licensing (as the other reply mentioned) is only one reason. Glib is not exactly known for high performance, and is significantly slower than the alternatives supported by the other languages I mentioned.

Glib us licensed under LGPL. So unless your project is happy with that, it's as if it didn't exist. That's one of the problems of having a small standard library.