Because there will be temperature limitations with PRNAV Arrivals and GNSS/RNPRnav approaches.
Because it's taking ages for the national authorities to grant approvals to do them.
Because not every aircraft is yet equipped with GPS and a capable-enough FMGC/FMS.
Because GPS is not accurate enough. GNSS/RNPRNav approaches are NPA.
How do you fancy doing a CATIII in 75m RVR with GPS? I don't.
Will that do?