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.
Exchange rate also goes down and up but still many businesses accept foreign currency and I believe it can also apply to bitcoin even if its more volatile. We can’t just buy and sell bitcoin, we need those businesses to use bitcoin and its blockchain technology or else this market is just a full of speculation. Altcoins on other hand already know their purpose and I’m happy that some businesses in my country are already accepting cryptocurrency as mode of payment, it will take more time before we see more businesses adopt bitcoin.