If it's so great, why hasn't it already been built?

Matt Asay has a post on 'Self indulgence and Silicon Valley' that gave voice to a question that I've been asking myself about Nimble which is, 'if this idea's so damn good, why 'hasn't' anybody done it already. It's a common question that every entrepreneurs faced with all the time. (10 seconds into the elevator pitch is when the eyebrows start to arch.)  Why? Because there are insulators that prevent people, even really smart people, from having the information they need to make sense of the world. How else could you explain George Bush.