Market forces lead to the creation of large corporations that then shut down market forces and undermine fair markets. Once a few big corporations dominate they coordinate their behavior and prices and shut down any new players entering the market. Regulation can counter it to a point, but once the corporations are wealthy enough to dominate government regulation also fails. Right wingers hasten the process by opposing regulation, and have no good answer to how to prevent markets collapsing into monopolies or cartels. I'm not sure anyone has a good answer to that in a capitalist system.