Most posts that are over 10 lines will almost always contain a TL;DR. I think the problem stems from the fact that comments are supposed to be short and so if you make a long comment without a TL;DR your post is not going to be read.

Another problem stems from the fact that people expect to get as much information with as little actual thought possible in a short amount of time; they always want new topics to stop their boredom. That stops actual discussions from taking place as in a few days people will have already moved on to another thing without enough time to have a think and create a discussion.

The structure of the website also makes it so that if you disagree with someone you can simply downvote them, and a -1 score will mean that nobody else will see your post. You could describe this as censorship, the 'hive mind' of the subreddit at hand will silence you for not following their exact beliefs and acting in the manner they themselves act. Of course this is not true for all subreddits, many of which engage in well thought discussion, but the website overall creates an aggressive environment for everyone else that detters any sort of argument.