Amram Tropper, in an article (PDF) on Rabban Yohanan ben Zakkai in the new issue of JSIJ, begins with this question:
In a famous rabbinic legend, Rabban Yohanan ben Zakkai flees besieged Jerusalem, surrenders to the Romans and heartens the Roman leadership by predicting their military success and Vespasianâ€™s promotion to emperor. This very same legend, in three of its four versions, also describes how Vespasian enabled Rabban Yohanan ben Zakkai to establish a rabbinic academy in Yavneh, the academy that would come to be viewed retrospectively as the central core of the burgeoning rabbinic movement. Thus the foundation myth of Yavneh, the story designed to describe the providential establishment of the rabbinic academy in the wake of the destruction of Jerusalem, risks depicting its central hero as a deserter, perhaps even as a defector and a traitor. Why would the rabbis have portrayed one of the most important sages of the formative period in rabbinic Judaism in this apparently unfavorable manner?
After a long 10 pages, he answers:
The missing element, the sufficient condition, which I believe explains why the rabbis were comfortable with depicting Rabban Yohanan ben Zakkai as opposing the war and fleeing Jerusalem, is Jeremiah. The rabbis internalized the story of Jeremiah and superimposed Jeremiahâ€™s role during the destruction of the First Temple onto Rabban Yohanan ben Zakkai.
Not such a good question, but his answer is interesting. He then proceeds to outline a comparison between Rabban Yohanan ben Zakkai and the prophet Jeremiah. It is because of Jeremiah’s precedent, Tropper proposes, the Rabban Yohanan was not considered a traitor for abandoning Jerusalem and his story was proudly retold rather than hidden.