According to Ubisoft, the game's delay is due to player feedback during the beta. Community developer Ubi_Irina posted on the official forum, "During the first phase of the Beta you were very active and provided us with very valuable feedback: your suggestions about general balance, factions balance and bug reports were very useful to us."
Ubi_Irinia continued, "Thus, we took the decision to delay the release of Might & Magic Heroes VI in order to secure a bit more time for the dev team to take into account your comments and deliver the best quality game."
Not all the news is bad, however. The beta will reopen on August 16 for all Might & Magic Heroes VI preorders, making the wait less painful.