Simple: owners thought games on free local TV would kill attendance. Thank the Redskins (Commanders, WFT) for getting the policy (it was never a rule) changed in 1973. In 1972, the Skins had a great year, got all the way to the Super Bowl, but those without tickets in the DC area were shut out from watching home games. Pressure from congress and even the Nixon White House, drove the NFL to put into effect a policy, where if a home game was sold out 72 hours before kickoff, the blackout would be lifted. Who determined if a game was sold out? The local team/owner of course.
Fast forward over 40 years and the policy was still in effect. However, in 2015, in a under-publicized move, the NFL declared the 72 hour policy dead and that all home games could now be televised locally, regardless of ticket sales. It was hailed as a good move for fans and teams alike because the more a team was on TV, the more people talked about them, bought t-shirts, mugs, bumper stickers, etc.