How Ali could take Foremans shots but not Frazıers so well?

[greynotsoold]

Every time Ali went down in his career (excluding wepner standing on his foot and pushing him down), it was from a left hook. Didn't see it coming.
A lot of fighters are like that. Carl Williams could take a right hand, but left hooks nailed him. Every time he was down was from a hook, but every time Holmes went down it was from a right hand.

[amat]

Pretty much summed up here but one thing that was also a contributing factor was that Ali never was able to grab hold of Frazier and absolutely neutralize Frazier. There was a very fair amount of holding but Joe never let himself get smothered like Foreman. Also, Ali was on his feet a lot more against Frazier which surely drained the gas tank. He also ran into a couple shots while moving, which obviously never helps. He squared up a lot to Foreman, more than he ever had before at least. Otherwise, everything everyone mentioned is essentially correct.

[boyla]

Every time Ali went down in his career (excluding wepner standing on his foot and pushing him down), it was from a left hook. Didn't see it coming.
A lot of fighters are like that. Carl Williams could take a right hand, but left hooks nailed him. Every time he was down was from a hook, but every time Holmes went down it was from a right hand.

That is true for most fighters, left hooks get more knock downs/ knock outs than any other punch.
It has a shorter distance to travel, can hit on the blindside.