There are an estimated 110.9 million television households in the USA. Cable has 65.5 millions subscribers. There are 30 million satellite television subscribers. I guess they figure the 15 million that still get free TV over the air, aren't the kind of fans MLB wants.
The thing that helped the Braves it the thing that ended up hurting them. Cable TV. WTBS was primed for the growth of the cable tv industry. The Braves capitalized off the success of WTBS' initial proliferation in homes across america as the cable industry boomed in the mid to late 70s. As cable tv further took root, in time most MLB teams came up with their own regional coverages, also on cable. For instance, given the choice in Florida or Colorado, why would anyone choose a shittily produced Braves snooze fest, when one could embrace the local heros with gusto and pride, still, while never leaving the comforts of home.