Must have missed that. The discussion I thought was about his seemingly quick adjustment to the NFL game.
He primarily fell because of the position change. Transitioning to the pro's is hard enough without basiaclly completely learning a new position, so you don't want to use a valuable pick on a guy who might not even be mediocre at his projected position. I agree with you, I think Fuller is a quality prospect but people are flying high on him right now. It definitely helps him that the Bears DC came up through the ranks as a DB coach, and many 4-3 teams have schemes that give the CB's leeway to make big plays, with lots of safety help over the top. I wouldn't have been upset had the Jets drafted Fuller (if they had gotten the opportunity) but I'm still a believer in Darquze Dennard, who is learning behind quality Bengals starters. I felt strongly about HaHa Clinton-Dix, Dennard, and Pryor as the top defensive players that would be available in the Jets draft range.
As another poster already stated I think the poor showing at the combine did him in. Even though he wasn't a SS in college, he showed the ability to play in the box and blanket TEs in college. That was good enough for me.