> Because Reddit is horrible.
You're absolutely entitled to this opinion, but I don't think it's fair or accurate to lump a site that is composed of almost 10,000 communities into one category or descriptor. It seems to me that categorizing Reddit as an "it" is only applicable when referring to Reddit corporate or the Reddit administration. Reddit is many communities with millions of contributors.
Take Yahoo Groups and give it a "front page" where you could access all the posts from the groups you subscribe to in one handy place and you'd have something very similar to Reddit - the difference being that most subreddits on Reddit are public posts and require no membership to view/comment. My point is that few would make the mistake of holding Yahoo Groups itself responsible for the culture of individual groups or say that "Yahoo Groups is horrible" because they disliked the content of individual groups.
I'm not denying that what you describe in your post can happen and does happen frequently, especially in default subreddits subscribed to by large numbers of people. You'll find that kind of mob rule in practically any large public community on the internet and it's ugly. What I'm saying is that such interactions are not the only experience that Reddit has to offer and there are plenty of places to have quality discussion and respectful disagreements.
The key is to seek out subreddits where that kind of culture is encouraged and actively moderated. If your front page on Reddit is made up of default subs, then yes, what you've described above is a pretty accurate description of the experience you're going to have. If you put some time and effort into subscribing to subreddits that favor quality content and interaction, your Reddit experience can be both enjoyable and rewarding.
Edit: Hah...and in all of that I didn't actually answer your main question; shame on me. My answer is that I found myself self-censoring or not contributing at all in larger sudreddits because of the prevailing culture of those groups. In smaller and more fairly moderated subreddits, I found the opposite to be true. I contributed more frequently and generously, forged beneficial connections with other members, and was mindful of the feelings of those I was communicating with.