There are lots of things you might consider, that would be impossible with snail mail. For example, the reviewers might be able to send regular emails with questions to the authors, or the authors might be able to constantly update the paper in response to initial reviewer comments, there could be an anonymous "chat room" of some kind, etc. Not sure if any of these are good ideas, but I think it is a fair point that most journals simply use the internet as a "faster snail mail".

That said, I review for conferences that are taking advantage of the internet, with author response to reviewer comments, discussions among the reviewers, etc.