I think it's that a lot of people find it overwhelmingly negative, "gossiping" about people on another forum, or just think it's flat-out stupid. But there is no mistaking that the thread's existence has driven people away from this board.
Here's my problem though: it's one thread on a message board. I'm not buying the negativity / stupidity because you simply don't need to click on that thread. If you're consuming that content, it's entirely on you.
Maybe the thread has driven people away from the board. I'd suggest that those people would've probably flapped their way out of the door if they read opinions they didn't like in threads they did approve of. If I were being a dick, I'd say 'fuck 'em'. If I were being charitable, I'd say that not everything is meant for everyone, so having to 'leave' a board is perfectly natural and not something that requires the board to change, when a user has already made the relevant change themselves.
The other thing I would say is that people on the 'very' left of the spectrum don't seem to appreciate that some people can still be resoundingly left whilst happily slaughtering sacred cows. There's an assumption that if you point to a subset of a demographic acting like cunts, you must be taking shots at the entire demographic. The old 'agree with me or you're alt-right' nonsense. That logic is wrong, simplistic, and should not be encouraged.
Ultimately, this isn't my community. These aren't my rules to uphold or my values to dictate. You cats do what you feel is best and let the chips fall where they may. I just hope that the 'winners' of this whole affair are considerably more valuable and gracious than that cat that ran straight into US politics to shitpost. Funnily enough, it's posters like that which have always made me feel unwelcome when I left the shitbin.