Because he was easily the biggest influence on Brock's career outlook, according to Brock. He told Brock to get into the business so he can get out, and Brock took that to heart. Chances our his lack of passion for the business would have still won out, but in his bio he cites Hennig's words so often it's clear they weighed heavily in his decision making.

The only other guys I see in his category are Austin and Savage. We saw Savage at his best for a long time though, and IMO he was criminally overlooked/under promoted in favor of Hogan, though with fair reason. There's no wondering about his career, and the more I think about it, the further up the ladder he goes and I'm kind of realizing he's standing just a shade outside that Rock/Hogan/Austin trinity.

Anyhow, Brock and Austin are the two biggest "what ifs" I can think of, because for all of Austin's greatness, his big money prime came after he broke his neck. If he never breaks his neck, he never loses his ability to work like an animal, and he's the unquestioned GOAT because he'd have been the only guy to embody every conceivable element that makes a great pro wrestler. He'd be marketable and money on the mic like Hogan and Rock, yet able to go toe to toe with the best technical wrestlers while having just enough size to believably brawl with the hulking monsters of the business.

Savage and Austin are the only other guys I can think of with that unique type of blend. The difference is Brock has the kind of intimidating presence that Vince cums over.