The renewed activity of re-attaching halakhot to the written Torah in Hillel’s time are due to the struggles of the times
The circumstances which brought about renewed activity in re-attaching halachoth to the written Torah at the time of Hillel are undoubtedly to be sought in the struggle of the Pharisees with the Sadducees, and in turn these circumstances necessitated the establishment of schools – the houses of Shammai and Hillel – who came occasionally together for academic discussions.
S.K. Mirsky, “The Schools of Hillel, R. Ishmael and R. Akiba in Pentateuchal Interpretation,” in Essays Presented to Chief Rabbi Israel Brodie on the Occasion of His Seventieth Birthday, ed. H.J. Zimmels, J. Rabbinowitz, & I. Finestein (London: The Soncino Press Limited, 1967), 295.