There is a lot more risk on the consumer allowing a merchant (and vicariously the consumer's bank) full "pull" access to a bank account. You'll need a dispute resolution system, which is what debit cards are basically providing in addition to point of sale systems. Imagine a security compromise like with Target, but if there was full bank account access as well... it would be a nightmare.

Bitcoin is "push" based which eliminates the ability to fraudulently pull charges from the sender's account. Banks also charge a fee, and add transfer times to transactions that are basically non-existant with bitcoin