Procrastination mostly?
In 2006, my roommates started their cosplay so late they spend all of day 1 of the con sewing in our room. I couldn't BELIEVE it....
This year I didn't want to start close to the convention because I changed my eating habits, so in a year I lost weight rapidly, and I was freaked if I did my cosplay too early it wouldn't fit right by the time of the convention. And I didn't want to take my wig out and style it too early cause I didn't want it sitting out around my house and get wrecked.
So I think people start later for worries close to their things getting wrecked, either that or they are honestly procrastinators. xD