Because negative attitudes have this tendency to overpower everything else in the room, textual or no. It's also incredibly difficult to follow more than one conversation in something like an LS due to the lack of line-by-line color segregation.
Maybe you've never been in this situation before, but trust me, this is exactly how it happens and it's not a good thing in the slightest.
I've been in this situation countless times and could still carry on a conversation just fine. An argument only turns into some big thing that engulfs the entire chat when people who aren't participating in said discussion cause a big stink about not wanting it to happen. I suppose I just lucked out and none of those arguments were strong enough to 'overpower' my unrelated conversation.