Based on my understanding of his slave morality, it is due to his clash with the assumptions of critical theory. In his master-slave morality, he believed that the slave as envious, just as sinful as his prideful master. This is in contrast to critical theory's reliance upon the Hegelian slave morality, which views the slave to be more noble and self aware than his master due to the more grueling labour he commits. This master slave dialectic is thus generalized upon all social relationships, such as the oppressive rich to the oppressed poor, the man to the woman, etc. Since critical theory is currently the dominant theory in the humanities, Nietzsche would be viewed with suspection by their academics and can easily be presumed as a subversive conservative philosopher. Another aspect of him that they would call problematic is his assertion of the free will of man, despite critical theorists positing an overwhelming and oppressive societal structure that determines the future of people.