Why is there such a lapse in activity regarding Torah Shebaal Peh from Sinai until Purim and the Anshei Knesset HaGedolah? What was the Oral Law like during this time period (I know that David and Shlomo Hamelech both made a couple decrees mentioned in the Gemara)? How is it possible that the transmission could have such a long gap?
The main reason, I believe, was that with the presense of the prophets, the need for such activity was diminished. What was once accomplished by the exhortations of the prophets to keep the commandments, was later accomplished with Rabbinic decrees. Another factor is that nothing was written down of Torah she'ba'al peh until much later, so the oral traditions of the first temple period were not recorded. The transmission was going on but possibly with less widespread study and legislative activity as developed later. (See Netziv's explanatioin of Megilat Esther)