April is the cruelest month because in a modern, secular context it offers no potential rebirth or rescue from the metaphoric death of winter. Whereas April usually carries with it the positive connotations of spring and the Christian Ressurrection, in The Waste Land neither of these events occur.
Many religions - Christianity included - contain variations of pagan vegetation myth that personify the seasonal changes in the actions and histories of various godheads. In the context of Eliot's waste land, where much of the world has adopted a secular, hyper-scientific perspective, these myths are reduced to an anthropological pattern rather than an authentic spiritual exploration.
April is therefore the cruelest month because one must stand in a postion that simultaniously observes a religon for its obvious patterns and cannot stand outside of this same system.