I don't know of anybody who's studied this in detail, though I would be curious to see the studies. I'm sure it has to do with the nature of the C programming community itself.
I think a large (maybe the primary?) part of the answer is: before the WWW, there was no such thing as a single resource for obtaining libraries for a particular language. People obtained their libraries, and knowledge of libraries, via many different means: through BBSes, mailing lists, newsgroups, and periodicals. The C community dates from this time, of course, and I've noticed a similar difference in culture regarding other languages from this period and before.
I think another part of the answer has to do with the general decentralization of C culture itself. There's no one C compiler, no one C development community, that serves as a hub and a potential point for projects to attach themselves to. And the C development community is huge, which further drives this decentralization and splintering.
In the case of C libraries, OS distributions actually do a pretty good job of collecting useful C/C++ libraries out there. (With the unfortunate exception of Windows, I believe.) They do a better job in these languages than most others, probably since C and C++ are such important systems languages on these platforms.
As far as CCAN goes, I think what would make a more worthwhile project, given the number of different distributors of C code out there, is to have a single site that links to the various libraries on their own native sites, rather than trying to get them to upload straight to CCAN. I think there's a use for this in and apart from Google, which will give you a lot of noise if you try just browsing for libraries. The question is, would you and the bulk of the C communities out there embrace such a site if it existed?
You might be amused to see how CPAN got its start: http://www.brainbell.com/tutors/Perl/CPAN_History.htm
CPAN evolved just as its community did. So the same thing could happen in the C/C++ world if the leadership and interest is there. But it hasn't happened yet.