They had to release a copy of the game for Evolution this year because it's slated to be featured as a main game for the tournament, the largest for SF in the world. The retail release was their original date and the special upgrade version for the US is earlier so players have time to acclimate to the game before it happens.
Want proof? The Japanese retail and upgrade versions are slated for August just like the US retail version. Of course, the JP market has the Arcade version but they didn't want to eat into their arcade profits unless they had to. They have to do release the upgrade for the US earlier or they risk pissing a huge part of their core demographic off. It appears at Evo but no one but the Japanese has had time to practice it? That PR shit storm would be incredible.
Oh, the timing makes sense, considering... despite being all of a month and change window of prep time.
But if was simply a case of getting it out on time, one would think that the costumes would be there too.
It may seem a trifling thing, but parity would be nice.