Because Ali stopped Foreman, Frazier, Lyle, Cooper, Moore, Quarry, Ellis, Foley and Liston and is much bigger and heavier than Dempsey while having big advantages in speed, reach and skill. Dempsey would get more punishment than he suffered throughout his career
Gonna reply in a general manner, Liston, Foreman and Frazier are terrible examples of Ali stopping someone. Liston quit in the first and arguably took a dive in the rematch, this is where the stereotype of Liston having no heart comes from. Frazier was willing to continue fighting, his corner stopped the fight against his wishes in between rounds. Both fighters were close to death as described Foreman had sus stamina and was beaten by tiring tactics that Ali used a good deal after his prime. At heavyweight, size and durability aren't necessarily the same. Chuvalo wasn't very big, but he's pretty much unquestionably the gold standard of a chin. Ali isn't stopping someone who was used to taking hard hits and would give him early trouble without giving up
Dempsey never took the kind of punishment in his entire career EVER that Ali would have consistently dished out. In Dempsey's era half the fighters who passed for heavyweight weighed in the low 170s and turned pro as middleweights. The men who were actually born and bred heavyweights were largely a mediocre lot that were any combination of unskilled, untrained, untalented, and past their prime. You can create all of the caveats you want for the great fighters Ali stopped but the fact remains that he stopped them. When Dempsey's resume of great stoppage wins is topped with guys like Willard, Firpo, and Carpentier this discussion doesnt even need to be taken seriously. Any number of lesser fighters on Ali's record would have slaughtered those guys as well to say nothing of the real legends. The one fighter who was anywhere near the combination of size and skill (and he wasnt even close) of Ali during Dempsey's reign was shamefully ducked by Dempsey for the better part of a decade. Anyone who thinks Dempsey would have fought Ali much less been competitive doesnt know his ass from his elbow much less anything about boxing. If Ali was active during Dempsey's career Dempsey would have starved himself to make 175 or he would have never been heard of.
Dempsey beat Willard, Firpo, Brennan and Miske so you're wrong! All were known for their tremendous skill, punching power, speed or some combination thereof.
Reeks of modernist bias, saying Dempsey wouldn't even be competitive with Ali is just a vast overstatement and highly ignorant. Iron chinned Willard is testament to his power. Again with the wills argument, Idk how many times that's gonna come up even though that was management. Dempsey warred with Firpo who went on a warpath proving his power, and Dempsey took that and then some, fired back with more power. Ali couldn't dish out that same amount of damage.
No, Ali described it as "the closest thing to death" he knew. That does not mean that he was literally close to death. But suffering from the volume heat & punishment involved. Frazier was virtually blind & at much greater risk.
Willis was almost totally inactive for years after he took the title. And softt for the Demspey fight. He had power, vut would have lost if not pushed back into the ring in time by the sportswriters. And was on his way to losing to Sharkey before the latter foolishly turned away to protest a low blow. Sure Firpo could hit harder than Ali in individual shots. But in terms of skill, combos, speed, defense...Against a top fighter Ali would normally do more damage.
Frazier looked close to being KO'd in the 14th, Liston did quit but was soundly beaten up in a way we hadn't seen before and Ali hurt Foreman knocking him down for the 10 count. Dempsey's not been in with an offensive force like Ali anytime in his career and was getting hurt by smaller men
Willard was a farmboy and Firpo a converted weight lifter. Their boxing skill was of a very low level and it shows how much boxing progressed after Dempsey when the talent pool became much deeper.
It wasn't just management, Dempsey himself said Wills would never fight for the title. Part of the reason he was innacitve for so long was because Wills was suing him through the courts to try and get his title shot. When Wills tried to block the Firpo fight, Dempsey said he'd never fight for the title.