I think it has to do with people struggling to find meaning in the absence of inherited religion or any other guiding philosophy. The forces of capitalism and democracy have stripped away many communal traditions, but people still have a need for some kind of meaning, so it's not surprising they might turn to a philosophy that is highly individualistic and doesn't require an organized church.
Disclaimer: I know very little beyond what I've superficially experienced. Not being a spiritual person, I had a hard time thinking common Christian systems made much sense. It seemed too much concentration was given to the hierarchy or building. I then met the Quakers. What I experienced there was much different. I'm not familiar enough with their customs so I'd rather not give bad information. I will suggest visiting a Friends Meeting if you have the opportunity.
