It didn't used to be this way. When did reason and the Left part company?
I think it has a lot to do with the theory that human nature is genetically hardwired to a degree and people's behavior can only be molded so far. This spells disaster for any type of social conditioning program that attempts to downplay the element of self-interest - an obstacle to the left's ideal world where everyone works for his neighbor's benefit.
Of course capitalism works because it has a realistic view of basic human nature and doesnt try to pretend otherwise.