Several reasons, including effects of scale on both sides (someone popular retweets a tweet of yours they don't like, and all their followers challenge you at the same time, or you are popular enough to get a constant stream of negativity), lack of moderation (no filter for low-effort flames from people who don't actually want a discussion), difficulty of having a debate in 140-character bites (the format in fact encourages aforementioned low-effort posts), etc.

But the main one is the blurred line between public and private. Posting here definitely comes with the expectation of a public debate and so people who aren't in the mood for it will not post. But on Twitter everyone is encouraged to publicly post everything from random status updates to highly personal matters to idle chatter that is meant to only circulate among friends (and may not be fully thought through). You can avoid this by setting your account to private, and many do, but this blocks the spontaneous spread of posts through the social graph by means of retweets and replies, which is IMO the best thing about Twitter. There is no granularity, either: only one account toggle between all your tweets being visible to the entire world or to just your direct followers. (Including replies, which makes private accounts an imperfect shield: if you have a private account but chat with a public account, people can still see their replies to you and infer the content of the conversation, plus your username.) Also, whereas you can easily leave HN or Reddit or some forum altogether if you get fed up with it, leaving Twitter may be tantamount to losing contact with friends; you can make your account private but it's not the same sort of mental break.