Senior U.S. counterterrorism officials cannot say for certain why al-Qaida chose today and an audio message by Bin Laden to raise its profile, but here is one analysis that is circulating:

Al-Qaida understood it had to do something to counter the Predator attack in Damodola last Thursday.  In the face of reports that significant al-Qaida leaders, perhaps even No. 2 Ayman al-Zawahiri, had been killed, al-Qaida needed to reestablish itself as a viable entity.

Having Zawahiri record an audio or video message might be too risky... if indeed he is still alive.  Zawahiri knows full well that his movements are being tracked. So for the sake of his own personal safety and al-Qaida's operational security, he would want to lay low until he felt more safe and secure. That could take weeks or longer. 

So with the Bin Laden audio tape in hand, it appears to have been recorded in early December, al-Qaida decided it would move ahead now with Bin Laden and wait for a Zawahiri tape. The strategy has the benefit of letting the American public know that Bin Laden is still alive and giving Zawahiri some breathing room until he has to make a tape... assuming, of course, that he is still alive.