The Ark was the provision by which survivors of the great flood were guaranteed a seat on that vessel if they would only climb aboard. It was not large enough to save everyone because God knew that only eight would survive, but it was large enough to have saved more people that it did.
The book of Enoch contains more information about the fallen angels and the Nephilim. This book was left out of the Bible because scholars doubted its authenticity.
So why bring a global flood? Such an extreme reaction was justified because of the condition of the people.
Violence flourished and the Nephilim were more interested in being famous than in being peaceful.
In today’s society, we still do much of what was done in the pre-flood world - the building of cities, development of musical instruments, the forging of iron and bronze for the purpose of making tools and ornaments. These activities had their start long before God ended that ancient world.
The human population were in awe of the angels that materialized in front of them and considered them Gods. They built the cities of the day with the help of humans. When people started to spread out on the earth, the angels taught them how to live in communes. While the Nephilim were violent and agressive, the fallen angels mostly took on the role of guides and teachers for they already knew the makeup of creation. Prior to the global deluge, the earth was just as enterprising as it is today.
The fallen angels that came to Earth tried to alter God's plan. They hoped God would forgive and accept the path they took. They wanted nothing more than to remain in human form and continue living on Earth. They intermingled with humans forming cult worship. In God's eyes this was unforgivable - those on the fringe were offered salvation in Noah's ark and God would direct them to Noah, but if they did not accept the offer to be saved out of a dying world, they would perish at the flood.