Mostly because C on a 16F part is pretty inefficient since the core was never optimised to be used with a C compiler. On top of that, I really only use 16F, 12F, and 10F parts when I need a low pin count devices for a relatively simple application. Since the applications tend to be uncomplicated, it isn't difficult to code the entire program in assembly.
The 18F cores made a number of improvements that allow for improved C compiler efficiency. On the rare occasion that I still use an 18F part, I will use C for the higher level program organization but will code any time critical routines in assembly.
I pretty much stopped using 18F parts in favor of the newer 16-bit PIC24s and dsPICs.