Or: because hashtags suck.
Let me qualify that: hashtags are a pretty good way of following what’s happening at a conference, but sometimes the most interesting tweets don’t get hashtagged. Tweets like:
- where people are going for dinner
- what they really think about that presentation
HeyLi.st solves this by building a Twitter list of everyone it knows is attending the event; just follow that list to see everything that the attendees are tweeting, not just the wry quotes they are hashtagging.
From “Introducing HeyList!”
HeyList! Homepage 
HeyList! Homepage