Hugh Grant and Elizabeth Hurley were able to come to Australia recently to spruik Mickey Blue Eyes because the schedule meant they could be in different countries at different times of release. This is particularly important if your film is leaning towards flopdom. A couple of stars schmoozing the local media can do wonders for box office. Indeed, staggered release may be the only hope your film has. Then there are simple issues of costs.
Film prints are hideously expensive.
The cost of making enough prints to open a film around the world simultaneously is too big a risk in most cases. Much better to share. Last, but by no means least, there is the seasonal nature of film itself. Some movies, particularly those with dense storylines or period settings, work best in winter. So, it's unlikely we'll see release schedules tighten up much more.