The Gemara in Masechet Megillah says:
אֶלָּא שֶׁהַכְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה׃ א״ר חֲנִינָא חֲכָמִים הקילו עַל הכפרים לִהְיוֹת מַקְדִּימִין לְיוֹם הַכְּנִיסָה כְּדֵי שיספקו מַיִם וּמָזוֹן לַאֲחֵיהֶם שבכרכין למימרא דתקנתא דכרכין הוי והתנן חל להיות בשני כפרים ועיירות גדולות קורין בו ביום ואם איתא ליקדמו ליום הכניסה הוו להו עשרה ועשרה לא תקינו רבנן
[The Gemara continues with the next piece of the Mishnah:] The villages, however, may sometimes bring the reading forward to the court day. Rabbi Chanina said: The Sages were lenient on the villages to allow them to bring the reading forward to the court day so that they can supply food and water to their brothers in the cities.Is this to say that it's for the benefit of the cities? but it was taught in the Mishnah: If Purim falls on Monday, the villages and large towns read on that day. And if it's as you say, they should bring the reading forward to the previous court day! This would bring it to the 10th, and the Sages did not enact the 10th as a possible day.
TL;DR: The Gemara asks, "Why can't we do this?" and it answers "Because the rabbis didn't establish it.
My question is, is that the question is "Why did the Rabbis not allow this?" and the answer is "It's because they didn't allow it." But the question is WHY the rabbis didn't allow it so this answer is not very satisfactory. IN summary, my 2 questions are 1)Why did the Gemara use such flimsy logic? 2) What's the real reason why the Rabbis didn't allow it?