I imagine it's one of those things where new equipment opened up new avenues of progress. First, there were no bench presses, and the only real chest work you got was from rings, and the only real lifting competition was all overhead stuff, so big shoulders matter, big chests didn't.
Then, people developed bench presses to help with the olympic press as an assistance lift. The early models were flimsy and comical, but as bigger pressers came along, better benches did too, which allowed for more progress on it. I imagine with bigger and better benches, we began to witness bigger and better chests, and soon we began to develop that association of strength/power with a big chest.
Powerlifting coming along would then help, because now we had a sport people could relate to on a lift that required less technical skill than the olympic stuff. Plus, you get to lay down, so that's cool.
We witnessed something similar with squats. Prior to the squat rack, squats were done with light weight on the toes and it was whatever you could get to your shoulders. Legs were well developed, but not massive. Squat rack comes, more weight can be moved, bigger legs come into play.
This is just my amateur understanding as a fan of the irongame, no real serious studying. Just remember stuff like this from Super Squats, Pavel, and the Complete Keys to Progress.