In short, vendors know they can trust credit cards. They do *not* know they can trust anonymous random joe customer who can so easily game bitcoin to their advantage.
Let me guess, you've never run a business before. Well I do. I trust bitcoins 1440 times more than I trust credit cards, even 0/unconfirmed transactions. Any credit card transaction can be easily reversed up to 60 days later. A bitcoin transfer is nearly impossible to revoke 1 hr after receiving it.
This thread is not about transactions that have received 6 confirmations, so don't even bring that up as it only confuses the issue.
What we are talking about here is 0 confirmation transactions, and if you really trust 0 confirmation bitcoin transactions more than a credit card transaction, then your enthusiasm has definitely outrun your business sense.
The solution is simple, and it's already in place with credit cards. Haven't you been asked to show ID when using a credit card? Solution: Accept 0/unconfirmed transactions with a valid ID.
And what are you going to do if someone shows you an ID and then double-spends anyway? Are you really going to go through the trouble of trying to track them down and sue them? Are you going to report it as a crime, an action that will with 100% certainty never get you your money back but maybe at least give you some personal satisfaction? If you do, how would you ever prove that the customer double-spent? The bitcoin address that was paid from is anonymous and cannot be tied definitively to the user. Furthermore, the payment address paid to (via the double-spend) cannot be proven to not be yours either.
Accepting bitcoins with zero transactions is just about the same, from a vendor risk standpoint, of accepting cash that has a significant chance of being counterfeit, and without any way to assess whether the cash is counterfeit at the point of accepting the money.
This isn't just rhetoric, I'm backing my words up with a real commitment, with real money and products on the line. You can walk into my store and buy a top-of-the-line computer and I'll let you walk out the door with it after seeing a 0/unconfirmed transaction on my bitcoin node and showing a valid ID. A credit card transaction is far more risky to me.
Awesome. Where are you located? I like free computers.