KBEAT Mainly because it has a much cleaner and more Mac like interface. Plus the fact that TweetDeck is an AdobeAIR app which makes it a resource hog. Most people don't want a Twitter client using the resources of Photoshop.
Also, when it was originally released, it was much closer feature wise with TweetDeck. Since then, Twitter's APIs have seen a lot of enhancements, and TweetDeck has updated with them regularly. Look for Tweetie 2 on the Mac soon, which will bring all the great additions that we now have with Tweetie 2 on the iPhone.
(Version 1.2.4)