Why didn't Noach Daven to save his generation like Avraham Davened for Sodom?
From Torah Insights for Shabbat Parshat Noach 5758 by Rabbi Aaron Borow:
From Jewish Values in a Changing World by Harav Yehuda Amital zt"l:
(this is similar to Monica's comment)
There is a popular idea (I don't know an actual source for it) that Noach was the classic "tzaddik in a fur coat." The metaphor used is that when the house is cold, you can do one of two things: wrap yourself in a warm coat, ensuring your own comfort but ignoring everyone else's; or build a fire (or turn on the heater, etc.) so that everyone benefits. Similarly, then, Noach was concerned about his own righteousness and that of his immediate family, but not so much about everyone else.
[It is true that, as the Gemara (Sanhedrin 108a) and other sources note, that Noach did reprove them throughout the 120 years that he spent building the ark. However, the Lubavitcher Rebbe zt"l explains (Likkutei Sichos, vol. 15, pp. 40-41) that he did so not out of real concern for them, but simply because G-d had commanded him to do so.]
That said, the Zohar (1:67b-68a) cites a discussion between R. Yitzchak and R. Yehudah on this point. R. Yitzchak indeed contrasts Noach unfavorably with Moshe, who prayed for his generation and averted Hashem's wrath against them; R. Yehudah counters that even Moshe was unable to find sufficient merit among his contemporaries, and had to appeal to the merit of the Avos - who of course hadn't yet been born in Noach's time. In another of his talks (Likkutei Sichos, vol. 25, pp. 21-22), the Rebbe notes that R. Yitzchak is not actually faulting Noach for his inaction, since indeed he was laboring under the constraint that R. Yehudah mentions as well as several others described in the Zohar there; he is simply pointing out that we should not use him as a role model.)