Primarily this is due to purpose and ease. Our operational networks, data and systems are not designed for public use. They are designed specifically for operational use. We have a high level of security we need to maintain for these systems and providing public access to them in a safe and efficient manner is technically complex.
AirNav Radar have readily available products, tried and tested with a variety of business users and the general public. They draw on a number of publicly available and commercially available data sources. These sources include Ground and Satellite based ADS-B, FAA SWIM, EUROCONTROL, MLAT, FLIFO, ACARS/Datalink and ADS-C. A large network of volunteer feeders in all parts of the world feed ADS-B data using ground receivers provided by AirNav Radar. The company currently has over 34,000 feeders who actively share data to the website. These sources are combined to create a representation of airspace that we consider as suitably accurate to support our communication and educational purposes through a widget.
Put simply it is much easier to use a widget designed to our specification by AirNav Radar with their data sources than to create our own from scratch. Furthermore, their data provides a good illustrative picture of airspace that is similar to ours. In fact when we compare our radar data with the ADS-B data of AirNav Radar, the positions are typically very close. The image below demonstrates this with NATS radar plots shown in green and ADS-B plots shown in white.

ADS-B (white) and Radar (green) plots