Why was it difficult to regulate large corporations and
monopolies?

Monopolies always reduce the economic wealth of society in many ways. Hence, governments regulate monopolies with the objective of benefiting societies more than would be the case if the monopolies maximized their profits.

The government may wish to regulate monopolies to protect the interests of consumers. For example, monopolies have the market power to set prices higher than in competitive markets. The government can regulate monopolies through: Price capping – limiting price increases

How can you regulate monopolies?

removing or lowering barriers to entry through antitrust laws so that other firms can enter the market to compete;

regulating the prices that the monopoly can charge;

operating the monopoly as a public enterprise.

missouri compromise of 1820 which prohibited slavery