According to Peter Beinart: http://www.thedailybeast.com/articles/2013/01/07/why-aipac-won-t-fight-hagel.html

It’s easy to exaggerate how big a defeat all this is for AIPAC. The Hagel nomination isn’t a good test of AIPAC’s strength precisely because it’s a cabinet nomination—a topic on which president’s usually get their way. It’s much easier for AIPAC to rally members of Congress behind resolutions that limit the Obama administration’s room to maneuver on actual policy questions, where opposing the president doesn’t look like such a direct slap in the face. (It’s also easier for the Israeli government to lobby Congress on policy questions like settlement growth and Iran sanctions than on cabinet appointments.) Furthermore, the Hagel struggle hasn’t been a complete loss for hawkish Jewish groups. His political near-death experience may leave Hagel more cautious when it comes to U.S.-Israel relations than he would have been otherwise (though I doubt that means he’ll turn hawkish on Iran).

Still, the Hagel nomination is a reminder that when the President of the United States decides he really cares about something, political realities change, especially for members of Congress in his own party. Democrats in Congress may want to stay on good terms with AIPAC, but AIPAC also badly needs to stay on good terms with Democrats in Congress. Nothing is worse for AIPAC, and better for J Street, than being perceived as a partisan Republican organization. And AIPAC’s leaders are smart enough to know that given the cultural shifts inside the organization—the increased presence of right-wing Christian evangelicals and Orthodox Jews—it needs to work especially hard to counteract that perception. I suspect that played a role in AIPAC’s initial decision not to make a fuss about the Jerusalem language in the Democratic platform this summer, and its apparent decision to punt on Hagel now. It’s possible that some right-wing AIPAC lay leaders will go rogue and lobby against Hagel on their own accord. Something similar happened in the 1990s when AIPAC officially backed the Oslo peace process but powerful AIPAC board members privately lobbied against key aspects of it. It’s even possible that by not fighting Hagel, AIPAC could lose some right-wing donors to further right groups like the ZOA or Bill Kristol’s Emergency Committee for Israel (ECI). But that risk pales compared to the risk of AIPAC becoming ECI or ZOA.