[Schtarflucz]

I think it's a matter of psychological leap (or the absense thereof).

Changing browsers requires aquiring new habbits. Most people don't like change. On the other hand, they're already familiar with the concept of a browser plugin thanks to Flash and Accrobat. The Google Chrome Frame can be understood as a speed booster and a bunch of additional features for IE rather than a new browser.

Edit: Icey burn me to it.