With Bitcoin and 21 you can set up payments within hours, if not minutes.

If you wanted to set up a traditional CC, you'd need to write the whole billing and tracking system, and so on. Managing all this is cost-prohibitive for smaller solutions.

Oh, and with CCs there are chargebacks, so you can never be sure that the money will actually arrive on your account. This virtually rules out any kinds of services where it costs you real money to run. E.g. if you want to create mechanical-turk API, good luck. Someone will pay with a stolen CC, ramp up $10k in bills, and then do a chargeback.