They don't like Twitch for being super user-aggressive. It's super hostile and will suddenly shove 3-5 minutes of ads at viewers unless they pay a few bucks for the month (per streamer), and then there's a lot of enshittification to disable if you don't want whacky nonsense like "Hype Trains" for how much money viewers shove at the streamer. No option to turn off ads either.
With YouTube they do have some enshittification, but most of all they limit privileges like adding links and whatnot if their automatic system thinks you're suspicious which is being under a certain threshold of views. Since DingDong doesn't post vods, he eventually got underneath that threshold, but he took the features being disabled as YouTube slapping him for naysaying a politician he didn't like given the timing. His cannel was also demonetized because the automatic system thought him browsing the XBox360 store was stolen footage, and they wanted him to record a live video proving it's his own content to get it reinstated.
The idea for his own streaming website was already cooking prior to this happening though. One of his favorite subjects is about how people should host their own stuff instead of letting the internet stay super centralized. The loss of some streaming privileges just vindicated him into finishing it sooner.
They also don't have to care about copyright flags and can just watch whatever. The realization dawned on them earlier this year and for 4/20 they hosted and watched a lot of anti-marijuana cartoons to poke fun at. On YouTube, even if they watched public domain stuff it'd get flagged.
DingDong also likes to show commercial reels and host radio too, so this site's convenient for that.