1. There is no discretion involved. When you're trying to finance a new development you have to convince bankers/VCs to give you money. Payment processors don't exactly need to be convinced, and Kickstarter is open to anyone.

2. The size of the cut taken by the middlemen is in direct proportion to the sales volume. With a bank you would normally take out a fixed-sized loan, then spend all the money on manufacturing, then pray that you can sell enough of your gizmos and pay back the loan. With buyer-financed manufacturing there is no guesswork involved.

Less discretion and less guesswork => less overhead.