I've never received a system update the day it was released. They roll it out, so it takes a while. I'm not sure why. It's not like Google's servers can't handle it. Youtube probably serves up more bandwidth in one second than all the people downloading 4.3 will. My best guess is that they're worried about bugs, so they roll it out in batches to make sure everything is running smoothly. That sounds like what a company of engineers would do