More to the point is why we are no closer to anarchy today than were Godwin and Proudhon and Kropotkin and Goldman in their times. Possible, but not probable.
Most anarchists are, frankly, incapable of living in an autonomous cooperative manner.
A lot of them aren't very bright. They tend to peruse their own classics and insider literature to the exclusion of broader knowledge of the world we live in. Anarchism as a milieu is not so much a challenge to the existing order as it is one highly specialized form of accomodation to it. It is a way of life, or an adjunct of one, with its own particular mix of rewards and sacrifices. Poverty is obligatory, but for that very reason forecloses the question whether this or that anarchist could have been anything but a failure regardless of ideology.