Why? There are many reasons, most of which relate to the usefulness of the posting. If, in my opinion, the usefulness of the post edges below zero, then we're into -- territory and the votes will be cast accordingly. Here's some things that degrade the value of a posting, at least in my opinion:

• Careless spelling mistakes or horrific grammar, excepting any made by non-native English speakers. Not casual typos, but sadly negligent, nearly wreckless typing.
• Failing to use capitals, or using only capitals.
• Making pointless pseudo-witty remarks which aren't funny or amusing.
• Use of pointless abbreviations such as "ur", "u" and so forth, including "34337 H4X0R" speak. Save these for IRC or ICQ please.
• Arrogance or refusal to admit defeat when the battle is clearly lost.
• Providing wildly incorrect or damaging advice, especially because of a rush to get the "first post".
• Posting dismissive remarks, such as: "use X", when there are many willing to take the effort to explain it properly. Don't bother.
• Missing the boat so badly that the post is actually counter-productive.
• Asking really, really stupid questions out of false laziness.

Nodes that are exceptionally worthless attract -- because it is our only practical way of protesting. It's harsh discipline sometimes, especially for some users who just don't get it (no names specified in the interests of being civil), but there aren't many alternatives.