Why is an ADS-B transponder not required in certain airspaces?

An ADS-B transponder is not required in certain airspaces, particularly Class D airspace, due to the specific operational regulations governing that airspace. Class D airspace is typically associated with smaller airports and controlled environments where the traffic is less dense compared to larger airspaces like Class B or Class C. In these areas, communication with air traffic control is necessary, but the requirements for an ADS-B system, which involves broadcasting an aircraft's position to other aircraft and ground stations, are not mandated. This allows for more flexibility and reduced requirements for pilots operating in less congested areas.

The other options do not accurately describe the regulations related to ADS-B transponder requirements. Daytime operation isn't a valid factor in determining the need for ADS-B, as the requirement is based on airspace classification rather than time of day. Similarly, there is no requirement for an ADS-A transponder as a precondition for having an ADS-B system; they serve different functions. Lastly, while aircraft below 18,000 feet may have fewer requirements in various airspaces, this statement does not exclusively apply to ADS-B transponders in Class D airspace.