Accepting Bitcoin also means restaurant owners are subjecting themselves to volatility risk. On this most recent plunge Bitcoin dropped some  33% in just a week. It then proceeded to rebound 23% over the next 5 days. That's some wild swings.

A restaurant owner that accepted Bitcoin one evening could wake up to find his earnings worth a big chunk less (or more) the next morning. He / she would have to be constantly converting Bitcoin to <insert local currency> to avoid volatility risk. They would find themselves becoming crypto trader... which isn't the business are in. Why do all this when they can just keep accepting cash and cards??