They had these elaborate math problems (that when calculated, did not add up correctly), and nifty graphs that looked, deceivingly, like someone smart made them.
Vatican II had reduced some people's faith in mainstream religion--how could true religion be exactly the same for so long and then substantially different all of the sudden?
The US had been in VietNam and there was lots of domestic unrest. The vast social changes occurring in recent years left those who had 1950s conservative values confused and feeling like something was missing. Hippies, Jesus Freaks, heroin--very disconcerting.
Environmental concerns were coming to light & people were concerned Earth could not be restored. People were seeking a solution, something different, something simple, so they didn't have to figure it all out themselves.
Educational levels and scientific literacy were lower back then, so it made for easier targets.