I know this isn't directed at me, and I think Luke might have some more insights that I'd like to hear. That being said, I think the main reason for it is that really the only people that want Bitcoin payment options are people who have Bitcoin. And that's a very thin slice of the population overall. The barriers for Bitcoin's success as a payment option are the same as Bitcoin as a whole... Perhaps the most relevant here is that for most people it represents the solution to a problem that doesn't exist. They can buy things online with their VISA or PayPal, so what is the advantage for the average consumer?