I'd figured the Extended edition wouldn't be out until Christmas time. If the regular edition was being released in may, they wouldn't put the EE out until at least 3 months later. At that point you are coming up on the christmas buying season, when people are more likely to pick up $75 gift sets, so they'd just hold off on it until nearer the present buying frenzy.
I am very happy that they will be adding the Mouth of Sauron back into the EE. That was always one of my favorite scenes. Pity they couldn't do the scene in the bar when the returning battle hardened hobbits face down "Sharky's" thugs.