Because a slick trim-bodied athlete at 6-3 and over would much rather try his luck in the NBA,NFL etc. than get his head pounded with minimal possibilities of being a millionaire considering not only having to face opposition but a corrupt governing system as well
I think a big part is that the HWs smaller than Wlad and Vitali are not in shape or solid HWs. Mike Tyson was a SOLID 218lber. He wasn't a soft 220lber would could probably tone down to crusierweight. HWs these days seem like Crusierweights who simply do not want to train hard enough to make weight and are "satisfied" at accepting being a soft and small heavyweight. Rahman is campaining right now in the 260-270lb range. I remember his best shape was a very SOLID 225-230lb. Chambers is a small HW who is fast, but would probably have better success had he originated at Cruiserweight