The reason? It’s not quite finished.
Overhaul president Trent Oster elucidated exactly why in the game’s forums: “Executing on our plan to make the best Baldur’s Gate possible and responding to that overwhelming feedback, we’ve added new characters, areas, and story, creating many new hours of gameplay. Yet the suggestions, fixes, and translations continue, and more volunteers keep joining.”
“After recently reviewing the game and then consulting with our partners, we’ve decided to ensure that Baldur’s Gate: Enhanced Edition is the best possible product on launch. Thus, we have pushed back the release date of the game. Our plan is to release in November, 2012,” said Oster.
This isn’t necessarily a bad thing. If it means a better game as a result, why not? We just hope that it’s ambition doesn’t exceed it’s reach.