I think there are a couple of factors that have contributed to the downfall of the HW division. Any other weight class you have to make weight, which generally means training hard, which inevitably increases your ability, which leads to a depth of talent in the division. No upper weight limit for heavyweight, coupled with more appealing options for bigger fighters who are actually prepared to work on their skills means the heavyweight division has become the realm of the freakishly tall or the fat n sluggish. This leads to dull fights that the public don't want to pay to watch.

The massive rise in interest in the UFC in the states over recent years means a lot of fighters who might have been heavyweight boxers have gone the MMA route instead, and once there's no american world champion (or contenders), there's zero american interest and zero american money. How many people in the US would have paid to watch Kliltschko vs Chisora, or have even heard of Chisora? That's why the lower weight divisions are richer in talent these days. Welterweight is full of quality american fighters who probably wouldn't have been interested in MMA since they don't have the size, and the PPV dollars generated are huge so there's a lot more incentive for talented fighters to get into it.