Why is a saltwater aquarium more difficult than a freshwater aquarium?

Maintenace of salinity and other levels such as trace elements, phosphorus, and calcium. All these are easy to maintain in aquariums 55 gallons and up and become quite stable once it is established. It is no harder than freshwater if it is larger (120 gallons and up) and stableized along with the right equipment. Once that is set up and has been running to get all the kinks out of it and all the levels are stable they are no harder than freshwater. This coming from a person with 7 tanks, two of them being reef aquariums.