Because it was released in England on August 30th and Australia on August 14th. America's release just got pushed back further and further. It was originally supposed to come out in June. I just bought the Region 2 so I didn't have to wait.
The reason why the America's version got pushed back on dvd is because Reign Over Me deals with the subject matter of one devastated man, who has lost his way in life after the loss of his family in the horrific 9/11 terrorist attack.