Satellite transponders for communications are expensive to maintain in orbit. Why not moon-based transponders.

Answer 1

Satellites stay in a geo-synchronous orbit, essentially maintaining the same spot in the sky at all times. always visable. Additionally, the moon is quite a lot farther away. The delay on voice and data transmissions would be enormous.

Answer 2

orbiting satellites - 18,000 miles away
moon - 262,000 miles
The strength of the transmitters and sensitivity of the receivers would make them prohibitive...
PLUS the moon isn't always visable rendering a moon based system useless