If you want a real answer to the question it would help to know French. Then you would go to Paris and try to find a Parisian of the age of 75 or over who was involved in the philosophcal-political "scene" of the 60s through 70s in Paris, and they might be able to tell you the real story. Failing that you might do some research in a French library while you were there. I am serious. This is the best way IMO. But difficult and not cheap!
Leo Loewenthal (Frankfurt School) made a few comments about it. George Lukacs wrote something on it I think, not much. Habermas. But these are not Frenchmen. The Marxian, and more open, writers have addressed your question. You are not alone in realizing this phenomenon. However, most of their remarks are not in depth. Sometimes you run across something in an interview transcript, but again it is not addressed in depth.
Keep in mind, the volume of Marxian books,writers, philosophers went down after 1979, particularly in America, and in Europe too.
The other thing is to look for dissertations (PhD.) On the subject, but avoid cultural studies and literary studies dissertations, because generally they are worthless for philosophers. Historians and French studies majors can be good,philosophers would be best. You may look through a really good research library for more current books. I am out of date myself. Look for books by philosophers. Also Journals in philosophy, French studies, intellectual history if you have access to journals. Oh, I forgot, Marxist, socialist journals, etc.
And of course, someone may come along here who can give you a good answer and good reference material!! But good references, really, really good studies, are hard to find which address your exact question, at least in my experience. You might have to write the book yourself.
PS. I think you will find that the earliest originators of postmodernism were "conservative". However, saying that does not capture the full flavor of Paris during this time. A person might switch from being a socialist to being conservative in one week's time, or he may have a string of ideologies at the end of his name.