The way I've seen it. Mike whacked the whole heads of the five families in one shot. I doubt too many people wanted to fuck with him after that. Roth was a jewish gangster and used his brain more than muscle but he failed. Killing fredo wasn't gonna make Mike look weak. I think new york didn't want to mess with mike since they knew how crazy he was. fredo could have been treated like shit and kept a close eye on but no reason to kill him