A crowdsourcing platform for bitcoin would require the website operator(s) to act as escrow for pledges due to the fact that you can't send a charge to someone for something like you can for paypal or credit card. (EG pre-approved payments, etc.)

This would require the website's operator(s) to be a trusted party. I'm sure a lot of people would love to volunteer for the job, but I'm not sure how many people would actually be "qualified" for it, so to speak.