A friend who works at FAA SATNAV office and evangalizes the move from terrestrial systems to RNP/WAAS/GBAS based procedures for Air Carriers have told me its all about $. The major carriers have pushed back on the $ to change (hardware, STC, Ops Specs, training, etc). Horizon and Alaska airlines both received significant federal funding to adopted RNP and GBAS precision approaches and traditional RNAV to WAAS LPV mins (still considered NP for alternate filing). Her office publishes a
quarterly newsletter about the progress here. They actually offered me a job in DC to draft ops spec, procedures, and training documents for 121 and 135 carriers and do road show demos with mobile procedure trainer type devices (similar to the PT devices used by air carriers - the ones you all use for learning the FMS and flows before starting sims)