The left has at its base the assumption that all people are equal. Not that all people should have equal opportunity, as the right in America believes, but that they are actually equal. That this is so obviously not true either physically or intellectually creates a problem for the left. The left also has issues with the role of science in society. The right believes that science should be used to overcome nature while the left believes that nature is more important than mankind. Here is a long article discussing this issue.