The main reason is that card and mobile payments cut down on handling costs, security risks, and the need for staff to manage cash drawers. Processing card transactions is way cheaper for restaurants than dealing with cash - no counting at close, fewer mistakes, and less risk of theft. Plus, there's the speed factor: card payments are faster during rush hours, which keeps lines moving and customers happier.
A lot of casual spots also saw this shift accelerate during the pandemic when contactless payments suddenly felt safer to everyone. Now that it's normalized, many places just stuck with it because the operational benefits are real. The data is cleaner too - restaurants get instant sales records and can track inventory better when everything's digital.
That said, you're not alone in finding it frustrating. Some places are definitely pushing it too far, and there are legitimate reasons to want the option (cash budgeting, privacy, or just what you're comfortable with). If it really bothers you, it's worth asking if they can make exceptions or at least pointing out the feedback - some restaurants do keep a small cash option if customers push back. The trend isn't universal yet though, so you might find spots in your area that still take both.