Probably the reason why people are not using bitcoin as payment because of its volatility. If you buy a shoes worth .01 btc or roughly $60 and tomorrow that .01 btc will be worth $80 so that you just paid $20 over the price of that shoes. So that won't really mean good for you. So i think bitcoin being so volatile makes people not want to use it as payment.
That can happen to you either when you buy some shoes or when you sell bitcoin, I don't see the difference. Besides, the price can go in one direction or in the other. It doesn't seem a good reason not to use bitcoin as a means of payment. The only one who could lose money is the seller, but for that there are the payment processors that allow users to pay with bitcoin and sellers to receive fiat money in their bank accounts.
That's happening already though fewer since bitcoin transactions are pretty much in an early stage still. Though because of how it's value moves up and down, people will always consider it as an asset.