That costs Kickstarter more money than waiting until the end.
If Kickstarter deducts in advance and refunds later for a failed campaign, then it must do two credit card transactions: once to charge the money, and another to refund it. Additionally, refunds are sometimes more expensive than charges.
Not only that, but Kickstarter would also have to worry about situations like, e.g., Alice pledges money, and then later closes her card account. How would Kickstarter get the money back to Alice?
So it's almost certainly done for reasons of simplicity and avoiding legal/ethical obligations about safeguarding the money in escrow.