Many reasons..... 1. His oppo at heavyweight was generally crap (Bowe and VK excepted). 2. He had the combo of speed and heavy hands. 3. He is getting old now, fighting at cruiser. Probably the worst ever two time champion - if you count the WBO.
I only really buy into the last reason you put down. His opposition at Cruiserweight is just as weak as his opposition at Heavyweight really, although he did get Bowe's attention with a few right hands in their circus of a fight. Edwin Valero is a two weight world champion, and he still hasn't stepped up the competition. He could turn out to be the worst ever, we'll stand by.
Just age. He's not as fast as he was and his timing is not what it was. If you watch him now he can barely connect properly he's so crude and telegraphed. The raw power is still there but its not effectively used. I've no doubt if he fought at cruiser during his prime he'd have been a proper champion. But he chose money in the heavyweights, and stayed there too long.
Agreed. But he wasted too much time on other meaningless fights when we all knew he wasn't a REAL contender at heavyweight against live opponents. In that respect he sacrificed achievement in the sport. He could have been a good cruiserweight.