It's simple. If you buy bitcoins with a credit card, you could then issue a chargeback. So the person who sold them would have the money taken away, and you would just keep the bitcoins.
Yes, but why do no-one makes a company that will make friend with a bank and tell them : hey guys, let's do that but check the blockchain before accepting chargebacks..
A credit card is just not the right way to do this. Non-refundable money transfers (wire transfers, Western Union etc) already exist and work fine.
It's going to take banks and their regulators many years to understand this "blockchain" you speak of.