The problem here is the content creators (*IAA asshats) who have no business sense. I can not get a good justification for the complexity of their distribution channels (country codes in dvd drives, IP restrictions, all of if). Why? Why?
It's a pretty simple and mundane explanation. Coordinating media (advertising, etc...) across the entire world is a big job, and probably pretty impossible to get everyone on the same page on exactly the same day for some kind of launch. Also, the stars of most movies need to make appearances for promotions, and they can only be in one place at a time. Rolling out different regions at different times allows you to manage that better.