In a monopoly, you are gaining an unfair advantage over any competition because you own so many infrastructures. Monopolies used to be known as trusts, which is why you sometimes hear of Anti-Trust Law violations.

At one time, AT&T owned every phone line, every phone and every piece of phone equipment in the country. They monopolized the industry; how could you compete with them when they owned everything? Similarly, the Post Office has an excellent infrastructure for delivering mail, but they do not have amonopoly because FedEx and UPS and DHL have all found ways to carve out a healthy piece of the parcel moving business, so although UPS always grumbles about the Post Office, they do OK in competition.