The Gemara in Megilah 4:1 says that if י"ד אדר falls out on שבת, you should learn about פורים on שבת. The Gemara then asks: "Why do we need to say this if we already know this from having דרשות on the שלש רגלים. מֹשֶׁה תִּיקֵּן לָהֶם לְיִשְׂרָאֵל שֶׁיְּהוּ שׁוֹאֲלִין וְדוֹרְשִׁין בְּעִנְיָנוֹ שֶׁל יוֹם: הִלְכוֹת פֶּסַח בַּפֶּסַח, הִלְכוֹת עֲצֶרֶת בָּעֲצֶרֶת,) (וְהִלְכוֹת חַג בֶּחָג! The Gemara answers: "One may think that it is prohibited from רבה's law about not reading the Megilah on Shabbos because one might carry a Megilah to Shul."
Why don't we ask "Why are דרשות allowed on סוכות? Maybe someone will carry their Lulav and Esrog to Shul?!?!"