Why cant commercial planes fly 800 mph or more normally?

There WAS a commercial plane that routinely flew over 1200 mph: The Aérospatiale and British Aircraft Corporation Concorde. That is to say, they CAN fly at 800+mph; it is technically possible and was historically accomplished many years ago (1969). Why they don't today tends to be an economic question and, to some degree, an environmental question, more than an engineering question.