Why did Lot make a 'Mishteh' for his guests, but Avraham didn't?
Seforno: 'Mishteh', by definition, means that wine was served, and Lot was fond of wine, as we know. Avraham, on the other hand, served wine at the weaning-party of Yitzchak, which was attended by the leaders of the generation. 1
Moshav Zekenim: Guests without arrive at night are thirsty from traveling, so Lot gave them to drink before eating. Guests who arrive in the morning are hungry, for they did not eat yet, so Avraham gave them to eat.
See Gemara Sotah, 10a.
Why did Lot bake specifically Matzos?
This conforms to the opinion in Chazal that explain Yitzchak was born on Pesach, since the angel informed Sarah that at this time the following year, she would bear a son (Refer to 17:26:1:1*). According to the opinion that it was in Tishrei, perhaps it is because Matzah takes less time to make, so he could serve them promptly.
Rashi writes that Lot baked Matzo because it was Pesach. Why, then, did Avraham not ask Sarah to bake Matzos when the angels arrived at his house?
Sifsei Chachamim: Sarah made Matzah Ashirah (kneaded with fruit juice), to honor the guests.
Moshav Zekenim: The Torah mentions "Matzos" only when talking about Lot, to indicate that he had Matzos (quarrels) with his wife (she opposed to his having guests).