Because chargebacks are relatively easy with credit cards and the merchant bank bears the cost of fraud. With Bitcoin, if you steals your coins, there's no way to undo that.
On top of that, there's all the convenience issues that plastic brings when out in the real world. Using bitcoin requires a computing device of some sort that's significantly less robust than Coin, a chip & pin card, or a piece of plastic.
it takes too long to confirm a transaction with bitcoin. Also the risk of having your bitcoins stolen is high - the reason credit cards exist is because reversible transactions are preferred to using cash. The credit card company acts as an escrow system.
I could imagine someone making a credit card that was denominated in bitcoin, but that would be a whole different technology stack than using raw bitcoins.
Because Bitcoin as a replacement for credit cards is currently just a pipe dream since less than 0.01% of people even know what it is, let alone have it. That plus its insane volatility is why it's only accepted by a handful of online merchants, mostly only for virtual goods and mostly just for show.
