Photobucket has none of the integration tools that made it easy. We'd have to allow for a url to the image to be input by the user and they could make it a huge picture, or a url to something bad etc.
Gravatars does the policing of the avatars with the ratings you use, allows us to define sizes easily and is already widely used by people meaning new users sometimes already have an avatar when they join (e.g. this guy
already had an avatar before we even opened the new site).
Plus as an added bonus are the default unique avatars. Avatars serve a usability purpose by helping distinguish users more easily, and users with no avatars can still display a unique avatar that is generated by a hash of their email. That's why even if you don't have an avatar you have a unique image on the site.
Originally, gravatars were intended as a temporary solution, until a better on-site solution was made but that may change as they work quite well.