One important aspect in addition to others that will doubtless be mentioned is that C is easier to interface with other languages, so in the case of a library intended to be widely useful, C may be chosen even nowadays for this purpose.
To take examples I am familiar with, the toolkit GTK+ (in C) has robust OCaml bindings, while Qt and Cocoa (respectively in C++ and Objective C) only have proof-of-concepts for such bindings. I believe that the difficulty to interface languages other than C with OCaml is part of the reason.
