When I took a tour a couple of years ago, the CM said that it's because of the fact that DL is more based around locals, and WDW is more based around tourists, especially from overseas. At DL, the changes bring people back at a different time of year. At WDW, if someone has saved up for years to come all the way to WDW and then can't ride the classic attraction they've been looking forward to because it's in its Christmas version, they are disappointed.