The reasons are varied… groups implode or become cults… people move on often times when kids get involved for all the reasons other have stated already… but also things get co-opted way faster now given less time for groups and movements to gestate and coalesce… but theres still groups like this and there probably will be in perpetuity…

they either implode or get so successful that they assimilate and morph… The early US was rife with utopian societies whose stories are legion, onieda and the mormons to name two contrasting early examples. More modern and infamous inline with the time period of this post is drop city… they got too successful as they were a stop on the way out to sf but they went onto do great things.