The main reason why nobody beat Marciano? It wasn't the fact that he was great, or that his era wasn’t exceptionally strong, or even that he was a good stylistic match for what was around at the time. What sealed the deal is probably that he was the most consistent performer that the division ever saw.
Guys, somebody sure would have "sealed the deal",...but there was always one problem,...Rocky kept beating the **** out of them....kinda spoiling everything, y'know?