As I understand it, it was (in short) about bad behavior. If they had only closed the 'politics' sub-forum at the time everyone would have just vomited their hate into 'general' instead.
In the end, Newgrounds 'as a place' is about growing as an artist, appreciating art, and having fun with games. There are plenty of other communities where people can discuss politics.
Tangerine
I remember hearing somewhere that it became too much effort to moderate for what it was worth. I guess also to incentivise people to use the supporter forum more??
But yeah, I was also pretty sad when the general forum was nuked.