While the story was obviously fake, the article and the tweets in particular have captured the attention of the mainstream media. For example, here's some commentary from The Washington Post and an article in today's New York Times. In addition, the Capitol Police is now investigating the situation and The Onion's false reports of the attack.
The Onion's reach on Twitter, (the satirical outlet had over 3 million followers as of August 2011 according to a report by Mashable) may explain much of the attention. Especially when you compare The Onion's Twitter followers to those of traditional print publications (from Mashable):