Because he couldn't beat Muhammad Ali in a boxing match, especially a prime Ali. It's that simple, really. In my opinion, to be the 'best' of anything that concerns fighting, you have to be able to defeat EVERYBODY, no matter what their size, strength, or weight is. Imagine someone walking around on the street saying they were the greatest fighter in the world or the baddest person in the world. You would naturally interpret them as meaning that they could beat anyone and everyone that they crossed paths with. But if they were to say I'm the greatest fighter/baddest person in the world even though I refuse to fight anyone that isn't my own size and weight, you would . Because that's not truly being the best fighter in the world overall then, is it? Ali was a HEAVYWEIGHT. So we know he can stand toe to toe with smaller boxers as well as the big ones. Can Floyd, at his size, defeat Muhammad Ali, whether it be the 60's Ali or the 70's Ali? If not, then how can be 'better?'