Besides their merits in the performance department, I've got a theory...
I've got an uncle who was one of the first bunch riding in Cape Town (South Africa), so he learned to kite a while back when people got lofted into buildings etc etc.
When I went over and let him try my old TD2 he HATED it, simply because of the lack of feedback and high bar pressure...
It's like most things I guess, you generally kite better on equipment that you're used to and are therefore inclined to continue buying that kind of gear (ie C kites)... eg: I HATE driving automatic cars... can't find the clutch!

. Even though from an efficiency perspective an automatic is far superior in the kind of city driving I do, familiarity forms a strong bias - which is equally practical.
It's plausable that there is some inertia among long-time kiters to switch to bows/SLEs, who are clearly more likely to be better on average than the latter group of kiters who most likely learned on bow kites and turned into 'bowmosexuals'.
How many learners these days vs. 'back then' are sold C kites?
Of those who aren't, how many go and try C kites?
Of those who try them out, how many are going to actually like C kites, which are chalk and cheese different to what they're used to.
What would be interesting to know is the average kiting experience and the kite learned on of those who fly C-kites vs. SLEs vs. bows... but then that would raise another question:
What is the greater determinant of a good rider? The kite type or the kiting experience?