"The 777 was the first time that Boeing went out of their way to ensure and validate the APU as a reliable system - because it was needed for ETOPS (especially 'early' ETOPS)."
Also with our intro of the B767-300ER and need for ETOPS we had to monitor the APU serviceability mostly, that it would start/run at altitude and oil uplifts were within limits. also a GTCP 331-200 and not the more powerful -400 unit.
Up until ETOPS the APU was considered a dead weight (unless you wanted MEL dispatch with a main geny out) and as such they were built rather light for the hard work expected on the ground..