Rav Eliezer Moshe Horowitz writes that the Chachamim actually hold that even b'Di'eved one may not read after Chatzos, and Raban Gamliel agrees that l'Chatchilah the time is until Chatzos. The Gemara was asking why the Gezeirah according to the Chachamim is different than the Gezeirah accoring to Raban Gamliel.

The Igros Moshe (OC I:86) suggests that there is an alternate possibility to explain the Chachamim, as the Yerushalmi explains. The Chachamim maintain that until Chatzos is mid'Oraisa. According to this, the Seifa of the Mishnah is the view of Raban Gamliel, and not the Chachamim. Therefore, the Gemara needed to prove from the Beraisa that the ruling of the Chachamim is also only a Gezeirah.

D. Zupnik