I'm trying to read up on some of the new developments in ATC (ATN, FANS etc.) and I've come across CPDLC: Controller–Pilot Data Link Communications. In particular, I'm confused of how the VHF messages travel to and from Air Traffic Control and the aircraft.
For the oceanic/remote flying, transmitting messages over SatCom sounds entirely reasonable. But everything I can find on VHF seems to imply that they use a separate Datalink Service Provider, similar how ACARS functions.
I always thought ATC had a bunch of antennas on the tower roof to do voice VHF. If this is the case, what prevents ATC from having their own datalink transceiver and doing all of this stuff 'in-house', avoiding more parties and infrastructure?