Because I have a 6 and 4 year old. If you have kids, I'm sure you're aware EVERYTHING takes longer with them. Getting up at that early hour to prep for the BG and then hitting some rides (standby and FP) will wear them out. Adding in hopping back and forth will just increase the chances of their (little) patience wearing thin (and understandably so). Also, some windows are 1 hour now.But why do you have to devote an entire day? Once you get your boarding group you can go anywhere and do anything. They give you a 2 hour window to get back and you can keep an eye on current groups to have an idea when you might need to go back. You could do any park, or have a resort day.