It's the grappling. Even in wrestling, HW's tend to look like two clumsy fat kids wrestling in their living room. For many of these guys, the grappling is just too taxing.Heavyweight fights should be limited to 3 rounds max even in main events. These guys gas out by the 1st minute it's ludicrous to have these guys fight for 5 rounds. It's a pain to watch for everybody.
HW's are made for short periods of throwing haymakers, and not much else.
Even the HW division in boxing is now being dominated by awkwardly large freak shows. The Tyson era was an anomaly.