We've had WireShark and other tools for a long time before. Why does everybody make such a big deal about FireSheep?
Ingredients of "success": Facebook + conference + FireFox. And sure, as @AviD has said - it is easy, no special knowledge is required. Good gathering of references is on Schneier blog: http://www.schneier.com/blog/archives/2010/10/firesheep.html.
Because it's easy.
Firesheep didn't create any new technology, and noone claims it did. But the point is, it has now become extremely easy to view and spoof others identities, and it is very accessible to almost any user (well, any user that can download and install Firefox and Firesheep plugin...)
It's really just a question of visibility.