People understandably don't want to make a second account to use a forum about something entirely unrelated to Twitch. No other forums does that.
It can be many reasons, really.
For some, including me, the forums have been declining in quality, and the whole Twitch thing was kind of the final breaking point, I'd imagine Twitch would want to make some changes to the forums if they're going all out with it, I really don't have any faith in those changes being good, either not really mattering or being overall terrible.
Others it's probably just a bias against twitch, with or without good reason. It's a pretty big company, and many people view big companies as generally bad or "evil", which is also understandable due to the behavior of a large portion of them.
Also possible that people just don't want to merge. There's weren't really any really good reasons provided for the merge, so I can see the hate against it.
Everyone has their own reasons, there happens to be a lot of them, so that's why it's kind of turning into a big deal.
Personally I feel it's not that big of a deal, but I'm already kind of over the forums and don't use them, so that's probably why I don't care as much.