Because monopolies greatly reduce or limit competition, and competition is the big force that makes capitalism work properly.
Capitalism without competition is no different from Communism or a lot of other systems in which one party rules supreme.
The main difference being that corporations divide up the different experiences of life, where fiefdoms divided the land. In the end we have absolute rulers in their own private domains who only have to pay a small tribute and lip service to the nation they are nominally aligned with.
Can you explain further?
Given that revised definition of "Communism," a greatly consolidated, monopolistic Capitalism, with increased favoritism by the state for leading firms, does start to seem similar in some interesting ways.