The real reason for the failure of a movement on the left is the lack of a common philosophy. Leftists of the marxist brand think that they can simply repeat the formulas of nineteenth century marxism and expect some result. And there is a large public involved with this perspective. Dozens of books are published on marxism every year (month?). But nothing happens. A new formulation is needed.
A good example of the problem is the question of darwinism. Both marxism and academic/scientific thought is stuck on this paradigm. But the reality is that thinking is shifting here. But under no circumstance will a groups as hidebound as the marxist begin to question darwinism. A whole way of talking from one side of the mouth arose here in the critique of sociobiology. But the basic critique of the theory as social darwinist ideology is beyond the left. And the left will simply dawdle as long as that is the case. There is a whole list of issues like this. Thus the marxist left is living in a kind of fantasy world. But the problem should be obvious: noone can organize a street protest with the classic canon, so it should be obvious something new is needed. There is not a rejection of communism. Far from it.