C2 is still a browser focused game engine, that alone puts people off by quite a margin (the node webkit issues we had alone would throw some people off).
then you have people working more into mobile markets and html5 markets as it is "easier", or perhaps quicker money wise, than having a fully polished game on PC, or because they want to legitimately target mobile.
there are also the ones who simply want to make games as a side thing and do not care about earnings, and since C2 is not a big investment in this realm, they can afford that.
there are also people I think using C2 as a first step in gaming developpement and then switching over something more conventionnal.
that would be my opinion on the subject, hope it is what you wanted.