I too, as a child of cult members, am very curious as to why people in the 1950s through 1970s fell prey to that kind of thing.
One clue I found is the occurrence of what are called (by serious historians) "great awakenings". These were periods of intense religious ferment, in which many new cults, sects and movements sprang up. Many of the fringe versions of Christianity we know now—such as Mormonism—sprang out of the Third Great Awakening.
The 60s and 70s are sometimes referred to as the Fourth Great Awakening:
Though the wikipedia article describes this as a Christian phenomenon, I think it parallels an importation of Asian religions, due at least in part to the Beat Generation.
One article I saw was that these "awakenings" were not demand driven: i.e. not driven by any kind of "need" on the part of the spiritual market. Rather, they were supply driven: a sudden profusion of spiritual offerings. This seems to be the case with the Indian gurus: they came on their own accord, into a fertile market. Similarly, J G Bennett functioned as an importer and promoter of new spiritual product from the East. Before either, Theosophy was the big importer of (what they claimed to be) Indian religious knowledge.
I think it boils down to: why do people buy anything? And how do they deal with growing evidence that they have bought an extremely expensive lemon?