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.
Although there is lightning network which can reduce high fees for bitcoin transaction but there just a few people who still interested to use the way.
It may lightning network is more centralized and make the user tend to avoid it. Whereas we need such platform, bitcoin's transaction is high already.
So, it will be hard if bitcoin used by online shop as mean of payment system. They may look an altcoin which has a low fees and fast for transaction process like XLM.