The leftish presupposition is that all people are equally able to be intelligent if trained/educated properly. Or at least, all races are equally capable. Therefore we must equalize or attempt to equalize educational attainment between groups

Leftists dislike IQ tests because they suggest that people can be classified as superior/inferior to others. This causes them emotional anguish due to their oversocialization and low self-esteem. They dismiss IQ tests as illegitimate because they are morally repugnant, not because they have a good rational reason for doing it (but theyll try to make up reasons to pretend their rationale isn't just emotive)

18 ...More importantly, the leftist hates science and rationality because they classify certain beliefs as true (i.e., successful, superior) and other beliefs as false (i.e., failed, inferior). The leftist’s feelings of inferiority run so deep that he cannot tolerate any classification of some things as successful or superior and other things as failed or inferior. This also underlies the rejection by many leftists of the concept of mental illness and of the utility of IQ tests. Leftists are antagonistic to genetic explanations of human abilities or behavior because such explanations tend to make some persons appear superior or inferior to others. Leftists prefer to give society the credit or blame for an individual’s ability or lack of it. Thus if a person is “inferior” it is not his fault, but society’s, because he has not been brought up properly.