The original point of cable was Community Antenna TV, where you'd get a much better quality signal (and often even additional out-of-market but nearby channels). Then broadcasters decided to go into specifically seeking nationwide coverage (Ted Turner was a pioneer in this area). They also decided, due to the sports leagues, that cable should only deliver local stations in the same market as your location through blackouts (through my childhood I went from getting three ABC affiliates and two CBS affiliates, to one of each). It became unprofitable to manage blacking out the out-of-market station any time they were both running network or sports programming, so the out-of-market stations were generally removed (I also wouldn't be surprised if negotiations for retransmission consent included terms preventing carriage of out-of-market stations).