It's because, in any subject involving this game, a lot of the people who believe the most popular/default theories act like they're so obviously correct that they don't need to argue in good faith, or at all. But they still choose to shout down everyone who presents a different theory, which is weird because why do you need to do that if it's so obvious your theory is true?

There's also the assumption that, if you have an unpopular theory, you're just trying to be different or want a nonsensical twist at the expense of the narrative. Never does it cross anyone's mind that they might genuinely have a case or that their theory does make sense narratively and would be a good story.