The reasons are numerous. AIM has steadly gotten more and more bloated, buggy, and generally a pain in the ass. DeadAIM fixed many of the lacking features in AIM, but it adds a new level of bugginess to AIM.
I have tried Gaim previously and found it too lacking feature wise and pretty buggy. Well I just tried version 1.0.3 and I am very impressed with how well it works so far. I am right now signed onto four different instant messaging services (AIM, Yahoo, MSN, Jabber) and not missing a beat. I am so impressed that I am going to give it a one week trial. If everything goes well, I will switch to Gaim full time for my instant messaging needs.
Great job Gaim team!