[Firaxus]

Keeping satellites in orbit around the moon is very expensive. The moons gravity is lumpy, and if you try to orbit far enough that it balances out, then perturbations from the earth and sun still end up destabilizing it. This means it takes a lot of station keeping (firing thrusters, using limited propellant) to maintain an orbit.

I think there are plans for better relays situated around the moon, but a lunar gps system is not likely given the costs and engineering difficulties.