Noah was a tzadik, a very great man. He saved the entire world. But he didn't approach Abraham's greatness.
When G‑d told Noah to build an ark before the world would be destroyed, Noah built an ark.
But when G‑d told Abraham He was about to destroy the cities of Sodom and Gomorrah—cities corrupt and evil to the core—Abraham argued. He said, “Perhaps there are righteous people there! Will the Judge of All the Earth not do justice?”
Where Noah donned a warm coat for himself and his family, Abraham built a warm fire for the entire world.
Because Abraham felt a sense of ownership for the world in which he lived. If there was something wrong, it needed to be changed. Even if it had been decreed by the will of G‑d.



I read the story as saying that Noah saved the human species from extinction and Abraham saved no one. He asked if there are 50, 40, etc., but stopped at 10, accomplishing nothing. If he had continued to 4, getting the same response at each number, he could have saved the city.