It's because they like Kerry but they're keeping it quiet. See this from the Jewish Telegraphic Agency:
And what's for AIPAC not to like about Kerry? The plan is that with the Arab states in complete disarray, Morsi out of the way in Egypt and Hamas severely weakened by the deposing of Morsi, Abbas will give Israel everything it wants. Ok he has no legitimacy but then nor does Israel. But the last thing AIPAC wants to do is spoil things by going all triumphalistic and it can't really feign complaint lest the more amateurish groups in the lobby misread the signals. So AIPAC likes the long face of Kerry but doesn't want to shout about it.