The answer is quite obvious. Professor Schlesinger's father, also a professor of history at Harvard, was a hard core left-winger of many years standing. I Schlesinger, Jr. practically cut his eye teeth on the socialist theme. The Schlesinger's, both father and son, actively collaborated in the writing of the now famous trilogy on the New Deal.2 Incidentally, all three volumes of this work were dedicated to persons who were leading left-wing figures.3

Any possible doubts as to Schlesinger's socialist bias were dispelled by an article he wrote for the left-wing Partisan Review entitled "The Future of Socialism: The Perspective Now." Here Schlesinger, Jr. explains his advocacy of Fabian socialism quite clearly: .

"Socialism, then, appears quite practicable within this frame of reference, as a long-term proposition. Its gradual advance might well preserve order and law, keep enough internal checks and discontinuities to guarantee a measure of freedom, and evolve new and real forms for the expression of democracy. The active agents in effecting the transition will probably be, not the working class, but some combination of lawyers, business and labor managers, politicians and intellectuals, in the manner of the first New Deal, or of the Labor government in Britain.4

Political murder called a "habit"
Although he favors a peaceful "creeping socialism", he looks upon the mass murder of millions in the Soviet Union as mainly a psychological problem of the murderers: "The habit of violence is hard to abandon; especially when it has worked in the past. A revolutionary elite always has the wistful conviction, based on experience, that it is easier to dispose of opposition by firing squads than by arguments.5

In the first statement, Mr. Schlesinger concedes that under socialism there may be "a measure of freedom" and new forms "for the expression of democracy". He assumes the new ruling class will be made up of the intellectuals and the professional element. This would naturally lead to a caste system in which class rule would evolve into hereditary succession. Parents usually fight to pass on to their children the positions and status which they have acquired. This is wholly at variance with the ideal of rule by the most capable. Schlesinger's admission that at best only "a measure of freedom" will be allowed in the New Order, combined with his tolerance of the Soviet's mass murders, recalls the old adage that "if you scratch a socialist you will :find a fascist".
https://exploringrealhistory.blogspot.com/2018/10/part-3-great-deceitsocialism-begins-as.html