That's a good question — and I'm afraid I don't know if there's a good answer! I think it's probably related to the fact that 腐 is an abbreviation for 豆腐. Characters with the meat radical are mostly about body parts, but some of them are about biological characteristics or biological action — like 腐, which outside the context of food actually means "to rot".

There's also the point about the radical not being guaranteed to relate to the meaning, but I think the above is a good argument. (It is, however, entirely my own conjecture so could be a load of rubbish.)