A professor in college once said something that stayed with me. He said that "bad ideologies will always find ways to self implode".
This stuck with me because it's true. Good ideologies tend to be pragmatic and flexible, and so they're able to adapt and evolve. However, bad ideologies tend to be more rigid and focus mostly on theories and ideals, and therefore they're unable to adapt or evolve. Far left ideologies firmly fall in the latter category which is why they are where they are.