I think part of it is the escrow model. If you preorder something directly from the creator, but not enough other people preorder it to make the project worthwhile, everybody could be in a sticky situation. With Kickstarter, you don't pay until enough other people also sign up to make the venture workable.
There's no doubt also some psychological stuff going on. Donating to Kickstarter in exchange for a finished product is probably much more appealing than directly preordering. In both cases you take risk, but in the former case you acknowledge it up front, and can feel good about helping the project even if they somehow fail. In the latter case, you'll just feel ripped off if they fail.
