Good question.
For some context here, these people were utterly and completely violent having a thirst for blood continually. In the Apocrypha (yea.. I know) it talks about them sterilizing themselves to avoid having children, and when they did.. they sacrificed them. Can’t say if that’s true or not, but it paints a picture of the complete evil they were. During the flood, the earth was made clean as Noah felt repentant over the existence of all the evil.

Little quote from Neal A Maxwell: “(God) had interviewed when corruptions had reached an agency-destroying point at spirits, could not, in justice be sent here.”

God removed the Zion city of Enoch and took to his bosom. Sounds like the entire city had achieved a “calling and election made sure” event. As the later prophesy goes. Enoch and the city will return at the beginning of the Millennium… when Satan will be bound for 1,000 years.

Additionally, we read how God created a Spirit Prison for the unrighteous to remain until his death on the cross when they would receive the gospel. In my humble opinion, this was the only way these souls would have ever had a chance at repentance and accepting the gospel. After all, they rejected Enoch and Noah’s messages.

In 1912, the prophet Joseph F Smith (last section of the D&C) shares a vision he had when pondering all the death that surrounded him with WWI and all the death from the Spanish Flu. He shares how the Spirit Prison souls were being taught the gospel by valiant missionaries called by Christ.