I think that Bellamy's work was so popular with the middle class for a couple of reasons. The first was that the world that Bellamy's future depicts is something so very far off from what reality offered in the America of the late 19th Century. The lack of jails and free access to public education for all are realities that are so divergent from the current configuration of American society at the time. In this, I think that some level of resonance with the middle class in the time period, a group that was so marginalized and pressured with the economic and social constraints of the time period. At the same time, the world of the future where there is little in way of social stratification and an more egalitarian point of view is something that is attractive to a group of people at the time period that found nothing but stratification. Bellamy's vision of a world in the future that is so much more desirable than what is present is another reason why there is so much embrace of his narrative by the middle class. The idea of projection of a future radically different and improved, by the standards of the middle class, is something that proves to be quite compelling for middle class people of the time period.