Aside from the excellent points that SethJ made in the comments here and to the other related question, we do find R. Yehudah Hanassi himself disparaging the use of Aramaic:
"Rebbi says: in the Land of Israel, why use Syriac [= Aramaic, see Rashi and Tosafos]? Either use the holy language, or Greek" (Sotah 49b and Bava Kamma 83a).
In the latter place, Rashi and Tosafos point out that the dialect of Aramaic spoken at the time in Eretz Yisrael was corrupt as compared to "purer" forms of it spoken elsewhere. So presumably R. Yehudah wouldn't have considered it a fit language for a major literary work.