According to a blog post by Perfect World Entertainment, Battle of the Immortals isn't just quite ready for it's debut. Instead of rushing it out the door, the company wishes to take some more time to iron out the finer details of the game, namely native support for Windows 7, 64-bit OS support, revised shop structure, revised AI behavior, and much more.
Because of all of this, the closed beta has been delayed until April, a full two months away from the original CB date, which was today. However, as their blog post says, "A late game is only late once. A bad game is a bad game forever."