It's two parts.Here's a random question. Why do people on Something Awful hate reddit? I think one sense, reddit is a more successful version of SA. It's one of the most popular websites in the world and even normies like to use it. Compared to SA which is dying a slow, sad death.
SA acts like reddit is only full of pedophile nazis. That might have been true ten years ago, but today people on reddit are pretty left. In fact, there are a lot of commie subs on reddit too like /r/ChapoTrapHouse. Of course, there are a lot of trans subreddits too.
1) Of all online communities, SA and Reddit are the most similar, which means there is the most interaction there and the most similarities. Ironically, they're both shitty software, for shitty communities, to write down shitty posts, which are in turn shittly moderated for a shitty leadership team to generate a profit. SA and Reddit don't like it when you hold a mirror up to them. There's SA and Reddit Let's Players, Trans communities, political discussions, WoW guilds, EVE Guilds, etc, etc and they're all full of basically the same people.
2) Even though they are similar, the being a "cool kid" on one forum doesn't make you a "cool kid" on the other. FAU/KM couldn't ban people they disagree with over on Reddit, so it must be garbage.
The only real difference between SA and Reddit is Lowtax, who is his own boss. Reddit has investors (and a valuation) which forces them to have an occasional surge of effort but Lowtax only reports to Barefoot Wine and Ambien. There may have been something of a rivalry ages ago but for most people SA has been 100% replaced by Reddit.