Because wait please explain to me the benefit of waiting 30 hours to make sure my order is confirmed instead of a few seconds using a credit card? Anonymity!! Guess what when bitcoin becomes "mainstream" which it never will for the very fact that it takes forever to confirm any normal transaction.
A zero confirmation transaction whose inputs are practically instantaneously validated against the blockchain is already beneficial to a merchant beyond any service VISA or any other payment system offers, because the credit card transaction is reversible at any time within a month. Waiting for the confirmations only protects against an attempted double-spend attack, which only is relevant in the real world proportional to the amount of money being spent.
I re-iterate, credit cards are already inferior to Bitcoin for point-of-sale transactions without ANY confirmations whatsoever.