(a) The RAMBAN and BA'AL HA'ME'OR (2a) answer that Abaye changed his mind
and accepted Rava's answer to his question.
In a similar answer, the RITVA says that Abaye is only listing the Tana'im
whom everyone (all of the Amora'im) agree maintain that a Sukah may be a
"Diras Keva." The opinion of the Rabanan, though, is subject to dispute
among the Amora'im, since Rava maintains that they require a "Diras Arai,"
and Abaye maintains that they also permit a "Diras Keva." Out of respect for
Rava, Abaye did not mention the Rabanan in his list of Tana'im who hold the
a Sukah may be a "Diras Keva."
(b) The RA'AVAD says that it is true that Abaye might hold that both Rebbi
Yehudah and the Rabanan permit a Sukah which is a "Diras Keva." The only
reason why Abaye mentions Rebbi Yehudah here is because it is clear that he
holds that a Sukah may be a "Diras Keva." The opinion of the Rabanan is not
clear -- all they say is that a Sukah which is 20 Amos tall is Pasul, but we
do not know their reason (as we find on 2b, the Gemara did not know the
reason for the argument according to Rav Chanan bar Rabah, who said that the
argument between Rebbi Yehudah and the Rabanan applies only to a Sukah which
is not large enough to contain one's head, most of his body, and his table,
but if it is large enough to contain his head, most of his body, and his
table, then even the Rabanan permit the Sukah to be 20 Amos high). Rebbi
Yehudah's opinion, though, is clear -- he certainly holds that a Sukah which
is a "Diras Keva" is acceptable.
(c) TOSFOS and the RITVA explain that even though, according to Abaye, the
argument in our Mishnah has nothing to do with whether a Sukah may be a
"Diras Keva," we know from other sources that Rebbi Yehudah holds that Sukah
may be a "Diras Keva." The Beraisa in Yoma (11a) and the Gemara later in
Sukah (21b) both clearly indicate that Rebbi Yehudah maintains that a Sukah
may be a "Diras Keva." Even though the Rabanan of our Mishnah, according to
Abaye, might agree and permit a Sukah which is a "Diras Keva," from those
places it is evident that Rebbi Yehudah not only permits it, but he
*requires* that a Sukah be a "Diras Keva," or that at least he was
*accustomed to* making his Sukah a "Diras Keva." That is why Abaye counts
only Rebbi Yehudah; the Rabanan *allow* a Diras Keva but do not require it.
Why, then, does the Gemara bring our Mishnah as the proof that Rebbi Yehudah
holds that a Sukah may be a "Diras Keva," and not the other sources? The
answer is that the Gemara (which is no longer Abaye speaking) is citing the
proof from our Mishnah only according to Rava -- to show that even Rava
holds that Rebbi Yehudah permits a Sukah which is a "Diras Keva," as we see
from our Mishnah. Abaye, though, would have brought the proof from the other