Why are 'opcode' field and 'funct' field apart in MIPS?

Question

MIPS ISA has an R type instruction, and the R instruction has an opcode field at its first 6 bits and a funct field at its last 6 bits. So why are the ISA designed like this? How about combine them into a 12-bits field?

Answer 1

My idea is that the three kinds of instructions share a prefix of 6-bit opcode. And for R and I types, the next 5 bits decide source register. If we combine opcode and funct for R instruction, the instruction format is not so consistent between R and I, which may make processor's design complex.