Bradman is there on stats alone; like yoblazer (an American) said earlier, he is the "stats" guy.
And it's not "most popular" or "better known" at all. That's a gross oversimplification.
There has to be a combination. There are very few who can really combine being an amazingly watchable personality that transcends their sport, draws new fans to the sport maybe, being the man everybody wants to see - while still having the stats etc to be in the argument. Ali may well be a technically inferior boxer to Robinson but if it is in debate, then what Ali did for boxing puts him over the top.
Bradman didn't have that transcendence or personality (as much, he was still a pretty eccentric genius if some stories about him are to be believed) but his raw stats are so insane, he's practically there by default. It is a similar story with Gretsky if memory serves.
There's a reason Ali is considered in GoaT arguments far more than Robinson or anybody else in boxing. I mean there are still those that consider Ali to be the best ever on a purely technical basis - TheRock and I had this exact conversation just a few days ago, in fact. The fact that a bunch of Robinson's contemporaries said nice stuff about him doesn't really settle that argument. But for me when you add on Ali's cultural impact, yeah, he's more fitting of the "GoaT" title.
Hell - you could even say that Robinson is the best boxer, but Ali is the better representative of Boxing in a GoaT argument, though that's probably overcomplicating things.
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