http://www.dafyomi.co.il/sanhedrin/insites/sn-dt-109.htmDaf Yomi / Sanhedrin 109
1) THE SIN OF THE "DOR HA'PELAGAH" QUESTION: The Gemara discusses the Mishnah's statement (107b) that the people of the Dor ha'Pelagah (the Generation of the Dispersion, which built the Tower of Bavel) have no portion in Olam ha'Ba. The Gemara asks what exactly their crime was. The commentaries point out that the Torah itself does not provide enough details to understand their sin, in contrast to the details that it provides with regard to the sin of the Dor ha'Mabul.
Why does the Torah not reveal what their sin was?
ANSWERS: (a) The BE'ER SHEVA explains that the Torah elaborates about a sin that was committed only in order for other people to take heed and not sin in a similar manner. Accordingly, the Torah describes the primary sin of the Dor ha'Mabul -- thievery -- in order to create a deterrent for that form of conduct. However, the sin of the Dor ha'Pelagah was unique in that it resulted from the entire world being unified and acting together. Such universal cohesiveness will never again occur on such a large scale, and thus no one will ever come to commit that sin again. Therefore, there is no point in elaborating on the sin of the Dor ha'Pelagah.
(b) Alternatively, the Be'er Sheva answers that the Torah does not describe their sin because they spoke in an extremely disrespectful manner towards Hash-m. It would be a Chilul Hash-m to relate what they said.
The CHIDUSHEI HA'RADAL in his commentary to the Midrash (Bereishis Rabah 38:11) also gives this explanation. He adds that, according to another explanation of the Midrash, the reason why their specific sin is not recorded is that they were at peace with each other, and in that merit Hash-m covered up their terrible sin.
(c) The MAHARSHA's explanation of their sin gives another insight into this question. The Maharsha explains that the people of that generation wanted to rebel against Hash-m but they were afraid of the possibility of another flood. They obviously did not believe Hashem's promise that He would never again bring such a flood. Therefore, they conspired to build a towering structure that would enable them to reach the heavens and empty the sky of its water. (See Maharsha at length.)
According to the Maharsha's approach, perhaps the reason why the Torah does not describe their sin is that it is merely a continuation of the sin of the previous generation, the Dor ha'Mabul. (Y. Montrose)
Dor Ha'Mabul = Generation of the Flood
Dor Ha'Pelaga = Generation of the Tower