It's not just "make sound investments," VC's need to make investments that will pay off big enough to meet their promised rate of return over the promised time horizon. (They are investing other people's money.) It's certainly possible for VC's to think that an idea has merit, but is not right for VC-type investment.

Kickstarter allows time horizons and rates of return of arbitrary size. These guys now already have everything a business needs: capital and customers. Unless they commit outright theft there is no reason they can't succeed with this project. But there is also no requirement that they keep it going after their last investor gets his or her watch. They can just do it to the size needed to hit the natural demand, and then call it quits. You can't do that with VC money.