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

[TitoFan]

Also Joe's beautiful left hook normally came out of the crouch position. Joe would launch himself up and into Ali's range. Ali preferred the style of defense where he leaned back away from the punches... and many of those left hooks went whistling by... but many hit the target. Joe being the shorter man... that helped also.

[Mars_ax]

Ali used the oldest trick in boxing, let your opponent punch himself out, and then when he's exhausted, you own him.

[Gandalf]

Classic fight, I always enjoy returning to this one.

Speed and timing were the difference. As has been mentioned Foreman really loaded up and you could see it coming, Frazier was just quicker and able to surprise Ali more.