Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
There's a solution to this, the automation to convert it into fiat/dollars as said by ranochigo. It's not that hard to understand the concept. The shops should be willing to accept it first and understand the basics of bitcoin so they are not walking blindly and just go with the go because bitcoin is the most popular cryptocurrency. They are accepting because they know that it's convenient for some people that owns it, not just go with the flow.