I think it has a lot to do with longer messages seeming "elitist" in addition to the tendency of trolls to find one phrase they don't like and derail the entire topic over it. You write 3 paragraphs, most don't read past the first sentence and vote based on that, and some troll starts nitpicking your use of "us" vs "we" instead of the actual topic. Over time you see putting the effort into a comment as pointless or outright adversarial, and you stop. It's the trolls and the low effort people that make having quality conversations frustrating. Not trying to gatekeep, but I firmly believe that once a site becomes popular enough that all the "Lowest Common Denominators" join, quality drops. The signal to noise ratio just becomes too much. Popularity is a death sentence on the Internet.