Schuller simply took advantage of the tremendous prosperity present following WWII. Orange County California experienced unprecedented prosperity beginning in the early 1960s.
I recall when his "church" was started in a failing drive-in movie theater (poor location). He stood atop the snack bar and the congregation stayed in their cars and hung the little
speaker on the inside of the driver's window.
During those years, you could just about arrive in California and fall off a turnip truck and make a success of yourself. So, he was the first to preach the "prosperity gospel" and the
"I'm OK, your OK" gospel appealing to the "Natural Man", the "Carnal Man" interested in self-gratification.
Now, prosperity is gone in Orange County California. It is in a "Great Depression" of sorts. I suspect it is reflective of the "Faded Glory" of what was once the Great United States,
a nation once "Under God".