Not your forefather's treaty

The rabbinic regime replaced the treaty at Sinai with the teaching at Sinai. The Torah (teachings) is uniquely Jewish, especially when the Torah is defined as the Talmud, whereas the brit could be co-opted by other sects, the Qumran sect or the Christian sect, etc. 

To define any aspect of the Torah as more central to the teachings than any other is to open up the question in ways that other sects could exploit. The rabbinic regime, therefore, from the outset, was much about the ability to distinguish themselves from competing ideologies at the time; and, as a result, the rabbinic regime never reached for comprehensiveness: it always strove for survival and internal solidarity. The rabbinic regime was not just about contending with loss of sovereignty and sanctity; it was also about defending against other pretenders to the rule of Israel.  

The shift of the first fruits liturgy from Shavuot to Pesach was, no doubt, a way of obfuscating the covenantal character of the Shavuot festival. Shavuot celebrates the first fruits of the land and emphasizes how it is that YHWH was the landlord Who afforded the land to the people of Israel by treaty. The rabbinic version transforms Shavuot into the celebration of the giving of the Talmud to the Jewish people, with barely a mention of the land or of any other term of the treaty.