The requestor wants to understand the reasoning behind either an operator's or avionics manufacturers business decisions with regard to ADS-B compliance but, does not specify which GPS receiver or provide any information on the aircraft or overall architecture of the onboard avionics. Answering accurately with specifics is hard.
On older transport aircraft early GPS equipage was done a number of ways to support different systems. One of the first uses of GPS was in the navigation system as a sensor for Flight Management Computers. On a lot of old jets, the ILS receiver was removed and multimode receivers that served as both ILS receivers and GPS receivers were installed.
GPS data are used by Terrain Awareness systems, ADSB, satellite communication systems and communication management units, ACARS, electric flight bag systems and it's captured by data acquisition units and safety systems like TCAS etc.
The SA aware requirement came out of the mandate to equip for ADS-B, which is essentially an ATC cooperative surveillance function. Those requirements are often met often by supplying GPS data to an evolved Mode S Transponder. Operational performance requirements have evolved and maybe still aren't universal across the globe (which would matter to a regional operator).
Compliance with ADS-B specifications involves meeting accuracy requirements, system integrity requirements and availability requirements. ATC must have visibility to those metrics to know how to space the aircraft.
In early industry discussions, suppliers and operators recognized that ADSB implementation with a stand alone GPS receiver on an older jet, could result in ATC getting better aircraft position than the crew had for navigation.
The use of GPS for aircraft navigation has steadily evolved from basic area NAV (RNAV) to GPS precision approaches and Required Nav Performance (RNP) procedures which can only be flown in an aircraft capable of meeting the Actual Nav Performance requirements.
Currently, the only practical way to meet the highest ADS-B and navigation system accuracy requirements requires, Satellite Based Augmentation System (SBAS), aka WAAS. Again, depending on which legacy units, upgrade could involve changes to receiver hardware, modified or additional antennas, and additional processing power.
Data requirements for all the different systems that use or capture GPS data are simply not all the same.
Some avionics manufacturers faced with investing significant sums in development and approval costs on a legacy product line with limited growth and marketing potential, found it logical from a business stand point, to abandon the evolution of their legacy product line and develop and deploy a new product with hardware support and processing power to meet all of the current requirements and have growth potential in the market.