In the past, Marvel would roll their films out over a two week period or so, with the US being one of the last to get it, because they wanted the good word of mouth to carry over to us to hype us up more and get more of us into the theater. At this point though, Marvel has reached a place in their popularity that they don't really need to do that anymore. The Marvel brand is so strong that people are going to see their films no matter what, pretty much.
Also, I assume some MASSIVE story developments and surprises happen in Infinity War and they (Kevin Feige mostly) want to avoid spoilers at all costs. Same day release across the globe will help with that for sure.