Great question, tpaine. For one thing, socialism deals with man only "in the mass" -- justified in this principle by Darwinist theory, which deals with him only at the species level. That is, it has zero regard for individuals (or of individual outcomes or "fates") other than as members of a class. Which effectively means: the individual is of concern to neither, for all practical purposes.
Darwinist theory feeds socialist theory its core principles in many others ways, just like momma bird feeding her chick. For instance, its "survival of the fittest" has been bastardized into "the class struggle" (as in Marx) or "race purification" (as in Hitler). Its "natural selection" puts blind chance (crudely put, luck) in charge of outcomes, not rational choice or personal morality. It justifies force, conflict, struggle as proper ways for man to ensure his survival, rather than negotiation and rational compromise. Man is never let free of his supposed status as a wild beast, who must "kill or be killed." In fact, neither "Darwinian" materialism or socialism has any room or rationale for human freedom at all.
Now in an all fairness, Darwin probably had none of this in view when he was developing his theory. But an idea whose time has come (in terms of broad, popular receptivity) gathers momentum and takes on a life all its own. Especially if some of the greatest mental manipulators of all times are constantly nursing it along (to their own advantage, of course).
Well, them be my views, tpaine. FWTW.