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.