Hello!
A monopoly was a large business that has taken over all competitors. Once the monopoly has taken over an entire industry, it has the power to regulate its own prices.
The American public primarily turned against monopolies because of their power over prices. For example, if a company were to take over a tissue industry, it could decide to sell the tissues for $20 a package instead of $5. The American public disagreed with this because if the people could not afford the $20 tissues, they could not buy the product from a different company for a lower price. They were stuck with a specific price from a specific company.
I hope this helps you answer your question! If you have any additional questions, feel free to comment or PM me!