I've read something a couple of months ago. Unfortunately, I do not find the source anymore but it was a discussion with Bob Arum (which I hate viscerally) but he said something that makes sense to me:

He says that the reason why HW division is faltering in the Us is mostly because now, the structures to professional sports are much more open than what they used to be before with very good supports for the athletes and such (which is true) and that it was draining a LOT of the top prospects who prefer to give a hit at the NFL or the NBA instead with a program of scholarship and special insurances instead of giving a try at boxing which is not as supported and much more risked. I think that it is effectively a source of the problem because then the pro leagues has to turn even more to what's going on in the olympian ranks. Such sportive structures aren't there in Eastern Countries, Hockey is well developped but that is almost it so their "big guys" potential is not drained by many high elite structures.