Many older homes were built well before air conditioning was widely available, so, leaving room for ductwork wasn't something the builders had in mind. To retrofit these homes with ducts would be a major disruption, not to mention costly. Unlike conventional systems, ductless systems take up very little space.
If you own an older home, chances are its historic charm is one of the reasons. Ductless can modernize the home's comfort while keeping the architectural integrity intact.
Both the National Park Service (NPS) as well as the Secretary of the Interior Standards and Guidelines for Rehabilitating Historic Buildings have recognized ductless-split systems as especially useful for retrofitting historic buildings because they are small and unobtrusive.