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.
Yes, if the store accepts payment in bitcoin, it will be quite risky. Because the value of Bitcoin is not stable, it fluctuates. Who is going to suffer losses if Bitcoin drops in price? Moreover this is very inconvenient for retail shops and even for shoppers. And for large businesses that can pay with cryptocurrency.