Re: Mike Tyson and Lennox Lewis bad weakness
About not stopping fighters late:
In Lewis' case, i think it's because he fights at a measured pace. He usually throws bombs early on to knock the opponent out or keep them at bay. If they can take them, then usually he'll box rather safely, throwing a handful of right hands per round. This makes sure he wins rounds and the fight, but will not often lead to a wear-down-knockout, hence no late knockouts.
Guys like Tua, Holyfield, Mercer and Tucker are known for having an extremely good chin. Hard to get a late stoppage against them.
About Tyson, the reason is because he threw his hardest punches in rounds 1-6. When his opponents were still standing after that they simply had the chin to take his best shot (Tucker, Green) or fought skilled enough to avoid taking too much punishment (Holyfield, Douglas, Lewis, Tillis, Smith although he held too much).
I think it mostly has to do with styles. Tyson emulated his style to Jack Dempsey. Dempsey only scored one knockout past the 8th, a 12th round knockout which should've happened earlier.
About not getting up to win, this is true.
However, this also has something to do with Lewis and Tyson being very hard to knock down. Lewis has been knocked down only 2 times in 11 full years fighting top contenders. I made a list recently and with career number of knockdowns and Tunney had the first place (but barely fought at heavyweight) and shared the second place in being floorest fewest times, together with Marciano.
Tyson has only been floored when he was pummelled for several rounds. The only exception is the knockdown by Holyfield in the 6th round of their first fight.
If both men had weaker chins, maybe Tyson would've been knocked down by Bruno, Tucker or Botha and Lewis by Klitschko, Bruno and Briggs. But in reality were hurt but not down, yet came back to win all those fights anyway.