A better question to ask was why was all of Noahs sons and wives allowed on the boat. If you buy into this theory then why didn't God just pick and choose?
Noah preached God's deliverance through the ark for one hundred years.
It is conceiveable that everyone in the populated world (at that time) had heard of Noah's preaching of the need to board the ark.
Everybody had the opportunity to board the ark (to escape the flood).
But, in the final analysis, ... only Noah's family (his wife, sons and their wives) boarded the ark in the time afforded by God.
Every other person on the face of the earth declined to board the ark (they, likely, thought that Noah was a flake).
God sealed the ark's door directly before the flood rains began.
As those first drops fell, and began to accumulate, ... others began to realize that Noah had been right all along.
But by then, ... it was too late.
As the scriptures tell us, ... it is vital that we respond in belief and obedience when God offers His salvation to us.
All those that respond so will be saved.
There were 4 very spiritual pastors who had a series of theological arguments on deciding just what the will of God was. 3 were always against the 4th. One day, after the usual "3 to 1, majority rules" statement that signified that he had lost again, the odd pastor out decided to appeal to a higher authority.
"Oh, God!" he cried. "I know in my heart that I am right and they are wrong! Please give me a sign to prove it to them!"
It was a beautiful, sunny day. As soon as the pastor finished his prayer, a storm cloud moved across the sky above the 4. It rumbled once and dissolved. "A sign from God! See, I'm right, I knew it!" But the other 3 disagreed, pointing out that storm clouds form on hot days.
So the pastor prayed again: "Oh, God, I need a bigger sign to show that I am right and they are wrong. So please, God, a bigger sign!" This time 4 storm clouds appeared, rushed toward each other to form 1 big cloud, and a bolt of lightening slammed into a tree on a nearby hill.
"I told you I was right!" cried the pastor, but his friends insisted that nothing had happened that could not be explained by natural causes.
The pastor was getting ready to ask for a very big sign, but just as he said, "Oh God...", the sky turned pitch black, the earth shook, and a deep, booming voice intoned, "HEEEEEEEE'S RIIIIIIGHT!"
The pastor put his hands on his hips, turned to the other 3, and said, "Well?" "So",shrugged 1 of the other pastors, "now it's 3 to 2".