I am no expert but a great deal of Leftism was attractive to people because they thought that it made sense. They had a few minor philosophers to go with and then some heavy hitters like Hegel. The only reason I am not a leftist myself is because of my own intellectual background which is more along the lines of the Talmud, Maimonides, Aristotle and then of utmost importance--Kant.  That is to say place Hegel on a lower level than Kant and also take the general approach of the people that developed Kant in a certain direction. That is not Hegel and not the Neo Kantians who took Kant in what I think is  a wrong direction. In any case in the long run I am saying Hegel did not prove his point and Leftism is based very much on Hegel. See for example the writings of Lenin on Hegel and also see Marx's Capital which is very much based on the labor theory of value -that something's value depends on how much work went into it.
In any case the Left used to have a case. But the judge of time and experience and Reason has thrown their case out of court.