or is it just cheaper ?
An "atomic" clock is maybe $2,000. I've not checked lately. Could be less. You can even get one that fits in your pocket (battery life may be an issue). This is why DTT, DAB, Mobile phone masts all use GPS. Obviously only for timing as they are not mobile.
One GHz terrestrial system I know has an oven based "clock reference" only in the main mast and then the clients lock to a pilot carrier for stability, to save £100 per unit.
The BBC used to distribute an atomic clock signal nationally.
A GPS receiver is so cheap! Just a few quid. Much smaller and lower power, so very many systems just take the easy way out and don't worry about the US turning it off or a big Solar Flare (which might fry all the satellites, will Sky refund customers?)