Being "simple" for a C program to decode is not the point; generating the smallest hardware to decode instructions and to reduce the critical path for things like deducing the instruction operands are.

For example, the operands all stay in the same position, which prevents putting decode on the critical path before you can figure out which registers you need to read (which is doubly important for superscalar designs which need to compute the register dependence graph). Likewise, the sign-extend bit for immediates is always in the same place, providing relief for another potential critical path.