Noah found grace in the eyes of God because God foresaw Noah's faith (Heb.11:7).
The same reason that Abraham did.
For I know him, that he will command his children and his household after him, and they shall keep the way of the Lord, to do justice and judgement that the Lord may bring upon Abraham that which he hath spoken of him. (Gen.18:19)
Noah and Abraham found grace because God forsaw that they would have faith.
The same reason anyone finds grace, they do not reject it when it is offered.
That is what Noah's generation did.
Well, that's one interpretation. The problem is did God see they would "work" for Him and declared them righteous? Isn't that salvation by works? Besides, faith is a gift from God so whose faith is it anyway?
We once talked about Abraham out on another thread. God appeared to Abraham and swore to make a great nation from him in Genesis 12:1 far before he was made righteous. It was in Genesis 15:6 where it says Abraham believed God and it was credited as righteousness and of course later for Issac where he was credited for works. If Abraham would have die at any time during this timeline would he have still been saved?
The other interpretation of course is that God, for whatever reason, chooses individuals like Abraham and protects them to accomplish His will. Abraham only had one purpose, that God was going to make a great nation through him. In my mind that's pretty amazing considering it is God who initiates the process. God came to Abraham and Noah, not the other way around. The scriptures says this is what happened. There is no indication that God looked down a "time tunnel" to see if they would have faith to the end of their days.