He was rocked and turtling up eating shots for extended periods of time not fighting back throughout the fight. If they were on the ground and that was happening the fight gets stopped you just don't see standing TKOs in MMA that often for some reason. Roy doesn't have some magical chin sometimes guys just don't go fully out JDS hit him harder and more often than Arlovski did. I think Mir neutralizes Roy's wrestling with his size and strength advantage.
I think the other thing is that Nelson respected JDS' striking so much that he was in a total defensive position the whole time. He had both hands up rhino'd, he had his chin tucked and his shoulders shrugged basically the entire fight. It's like how fighters are on the ground against Maia. They don't even try to get up or go for subs they just concentrate 100% on defending. That's how Maia has no problem easily controlling them the entire time on the ground but hasn't got a lot of subs as of late.