The following was taken from here and expanded upon
C is easier to program in, compared to Assembly. There are obvious reasons not worth rehashing.
Being easier to use, C allows you to write programs faster. Generally these programs are also easier to debug and easier to maintain. Furthermore, it's easier to manage large, complex programs in C.
Often times, code generated by a compiler is equally as good (in terms of speed and efficiency) as hand-written assembler - if not better.
C is pretty darn low-level, and it's rare that you'll want to go much lower. Having an added layer of abstraction is rarely a bad thing.
When you do need to go lower, you can use Assembly, otherwise you can use C.
You can write Assembly in C-code, but not C in Assembly-code.
A C compiler applies optimisations reliably and systematically - not just when it notices the opportunity
Creating a miner is often done in a collaboration and there are far less assembly programmers out there
C++ (which extends C) and is popular with miner development further improves development time, organization, and collaboration