Rabbenu Bahayye (Bahya) states that the question begging to be asked, is why didn't Noah, who is described by the Torah as being righteous, pray on behalf of the people of his generation to save them. We see that Abraham Abinu 'a"h, prayed repeatedly on behalf of the wicked people of Sedom (Sodom). Indeed, we see that the Prophets and others prayed on behalf of their generations.
The answer is that Noah was not remiss in not praying for the people of his generation. Rather, it was because Noah understood that for the world to exist there must be at least 10 righteous people, as Hazal (our Rabbis of Blessed memory) tell us. Indeed, G-d would not have brought the Mabbul (the great flood) on the earth had there been ten righteous individuals.
Infact, the only people permitted to enter the Ark were Noah and his family, who were a total of only eight. As a matter of fact, Abraham Abinu also stopped praying to G-d when he realized that there were not ten righteous people in Sedom. Not only that, but the people in Noah's time had been given 120 years warning to change their ways (which was the time it took to build the Ark), but paid no heed.
(See Rabbenu Behayye on the Torah, 6:14)
