Maybe because people tire of arbitrary off-topic comments that don't actually have any substance. No interesting take, no obscure link, no analysis.

Have you ever seed a headline that reads "Map found in attic" and the first post is "I read that as Man found in attic hah"...and you wonder what the point of even posting that was?

Obviously, this is a social media website with a fair amount of watercooler chat, not something super-rigorous like /r/AskHistorians. But downvotes are a way to filter out useful content from non-useful content. They aren't saying "you are wrong to think that way".