In its simplest form the philosophical basis for Karl Marx and of communism is that the prime mover of society is economic imperatives. In other words a population tied to the land is an agrarian society because farming demands a certain type of society. It is a fairly reasonable hypothesis.
Economic imperatives change over time and for example the economic imperative that made the social convention of marriage a necessity - a man working the fields and a woman caring for the home, has ended. Marriage is no longer a financial necessity. Men and women have moved to an equal income, child rearing is no longer a necessity and so on. Marriage, as it has existed for the last few hundred years, is not appropriate and a new sort of relationship that is sustainable in a communal society must emerge.