i like this question, so i will give it a shot. i will likely edit/add to this answer.
it is my experience that there are at least 2 reasons folks bail on living a radical life.
1. their radicalism was merely a phase. this may sound somewhat dismissive, but i don't mean it that way. i think the extent to which the acceptable values of modern human life (mass society, education, job, home, kids, rinse and repeat) are ingrained/indoctrinated into everyone makes it extraordinarily difficult to break free of. eventually those values might win out over the continued struggle for a liberated life; what it means to be "free" becomes adaptable to those values.
2. many, many folks are convinced that the only way they can live a radically different life (one that does not adhere to or prioritize those values mentioned above) is to "change the world". mass social change is seen as the only path to a truly radical life. once they realize - as most will, at some point - that changing the world to match what they want it to be is impossible (no way to change others) and/or undesirable (there is no one "right way" for everybody), they become disillusioned and revert back to some more "practical" way of being in the world, which typically means allowing those values to regain prominence at some level.
my own disillusionment led to me bailing on the "mass movement" concept (and the acceptance of mass society as some requirement or ideal), and invigorated my desire to create my own life for myself, prioritizing my own values (as much as i am capable) over those that have been imposed.
i also think people's perception of what constitutes a "rebellious" life, what is "success" (in terms of being a rebel), and related concepts, all can play into how one deals with the inevitable frustrations of desiring a life outside the status quo.