James_woods:
"I think they quickly discovered that national media (like radio or paid print) COST money - while getting their people to go door to door and sell literature MADE money."
I disagree. Many preachers manage to fund their radio and television broadcasts. Joseph Rutherford kept up his radio broadcasting until he was forced off the air.
In my opinion, it's a matter of direct control over their members. In George Orwell's fictional totalitarian state, 1984, mention is made of the fact that the Party was keeping its members constantly busy through a wide variety of forced activities other than work. The purpose of that was to keep them from having the spare time to think or do anything that might lead them to develop an independent state of mind.
Villabolo