It says in Masechet Megillah
אָמַר רַבָּה בַּר בַּר חָנָה א״ר יוֹחָנָן זוֹ דִּבְרֵי ר״ע סְתִימְתָאָה דְּדָרִישׁ זְמַן זְמַנָּם זמניהם אֲבָל חכ״א אֵין קוֹרִין אוֹתָהּ אֶלָּא בִּזְמַנָּהּ מֵיתִיבִי א״ר יְהוּדָה אֵימָתַי בִּזְמַן שֶׁהַשָּׁנִים כְּתְּיקְנָן וְיִשְׂרָאֵל שְׁרוּיִין עַל אַדְמָתָן אֲבָל בַּזְּמַן הַזֶּה הוֹאִיל וּמִסְתַּכְּלִין בָּהּ אֵין קוֹרִין אוֹתָהּ אֶלָּא בִּזְמַנָּהּ רַבִּי יְהוּדָה אַלִּיבָּא דְּמַאן אִילֵימָא אַלִּיבָּא דר״ע אֲפִילּוּ בַּזְּמַן הַזֶּה אִיתָא לְהַאי תַּקַּנְתָּא אֶלָּא לַאו אַלִּיבָּא דְּרַבָּנַן וּבִזְמַן שֶׁהַשְּׁנַיִם כְּתְּיקְנָן וְיִשְׂרָאֵל שְׁרוּיִין עַל אַדְמָתָן מִיהָא קָרֵינָן תְּיוּבְתָא דְּרִבִּי יוֹחָנָן
Rabbah bar Bar Chanah said that Rabbi Yochanan said: This rule stated in the Mishnah are the words of Rabbi Akiva, the anonymous author, [He's called this because when Rabbi Yehuda HaNasi compiled the Mishnah, and didn't mention the name of an opinion's author, it was usually Rabbi Akiva's opinion.] who interpreted 'time', 'their time', and 'their times'. But the Sages say: The Megillah is only read in its time [the 14th and 15th of Adar]. They raised an objection: Rabbi Yehuda said: When does this rule apply? When the years are properly fixed [when there's a Beis Din that fixes new moons and leap years], and the Jewish people inhabit their Land. But these days [literally: at this time], since people look at it [they count 30 days from Purim to Pesach, since Adar always has 29 days], it's read only in its time. Which opinion is Rabbi Yehuda following? If I say he's speaking according to Rabbi Akiva, this can't be, because according to him the enactment [that the Megillah may be read on the 11th, 12th, or 13th] is even in effect until today. Rather, isn't it that he's speaking according to the Rabbis, and even according to them in any event, we read on the other days when the years are properly fixed and the Jewish people inhabit their Land. This is a refutation of Rabbi Yochanan.
The reason why we reject R' Yehudah's answer is because it doesn't follow R. Akiva or the Chachamim's answer. But why can't R. Yehudah have his own opinion?