RF: The Rishonim ask that exact question (actually, the Gemara itself might ask but I’m pretty sure it doesn’t). The standard answer is that Sukkos and Pesach require 30 days because they have a lot of Halachos but everything else is just on its day.
If I recall correctly, and I probably don’t because I never saw most of it inside and haven’t discussed it in over 10 years now, the Meiri at the end of Megillah quotes a different answer (D’oraisa vs D’rabannan) and someone brings down that there is a Din D’Oraisa to learn Inyano Shel Yom but an additional Chiyuv D’rabannan to learn about things with lots of Halachos 30 days before as preparation.