Reason #3 is the main issue I'm going through now. A normal bulk is about 10-20lbs, but a lot of guys still do the Lee Priest / Kevin English bulk, where the drop can be 40,50,60lbs more. That's one heck of a gain/drop in a year.
Reason #8 is on the money, but lean guys need to be mentally fit. A lot of guys can't stand looking small (or tiny). Even peeled, ripped, dry, they're miserable b/c they can't lift as heavy as the big boys. I'd say #8 is key, but I'd add that psychological factor for good measure