Varieties of criticism
Conservative criticism has nothing much to do with "left-wing" or "right-wing", because left-wing people are often very conservative, in the defined sense, while right-wing people can also be very radical, in the defined sense. The difference between "radical" and "conservative" has more to do with the belief in whether "a change to something genuinely new" is really possible and necessary. Radicals typically believe strongly that such change is highly desirable and necessary, and that it can be achieved. Their criticism is that there is "not enough" change. Conservative critics, by contrast, are very skeptical about any such change, because they feel the change is really "just another form" of something that already exists.
Manning Clark
By the 1970s, Clark, while still writing history which was conservative in a historiographical sense (that is, not based on any economic or class theory of history), had come to be seen as a "left-wing" historian, and eventually he accepted this label, despite his fundamental scepticism and pessimism. This meant that left-wing intellectuals and commentators generally praised his work, while right-wingers increasingly condemned it, in both cases often without much regard to the merit of the work.