...because C is more structured and clean language. C-compiler is more "clever" and is able to find many typpos and other errors in your code due C-language rules is more strong (oh, but it cannot find own errors :)
Also, using many already-done libraries it is simple and more fast to create new product.
From other side, with good knowledge of ASM you may produce faster and shorter programs than C does.
So, summary: use ASM for time-crutical routines. The good solution: create separate .asm file, put all your low-level assembly routines to there, make these functions as public, and call them from main C program, where you declare them as "extern". Due compilation, linker creates hex from both C and ASM object files - no problems!

Regards,
Oleg