Because all positive discrimination is negative discrimination to someone else, so obviously when you are against discrimination you should be against positive discrimination too.
How do you know when you have enough positive discrimination?
Do you just make sure that the representation of different groups is the same as in the population? This is a poor solution because it cannot be said that this would have been the distribution without negative discrimination.
Also, along what lines would you now pick your categories?
Would you ensure representation for latino lesbian women who speak spanish and like chocolate cake on tuesdays?
Which is to say that the category can get arbitrarily small.
Those who do the former, point at decreasing gaps between population groups as evidence that their approach is more fair. Those who do the latter point at specific cases where a person was ipso facto denied something that they would otherwise be a better fit for, because they were positively discriminated on the basis of some factor.
The catch is that conceptualization of fairness is driven by empathy ("I wouldn't like being treated like that if it were me!") - and empathy in humans works towards individuals, not large populations. A guy who couldn't get a job because his race made him rank lower is easier to empathize with, because the injustice is done to him personally and explicitly, as opposed to another guy from a historically discriminated group who is suffering from an accumulation of subtle but systemic biases throughout society.
Thus, too much or too visible positive discrimination triggers a significant social pushback, which may well swing the pendulum far in the opposite direction.
But you're missing the point.
All discrimination is aimed at individuals. Claiming that because individuals of group A have been negatively discriminated against, you don't balance it out by performing positive discrimination for them (even if it is exactly the same individuals), because you achieve that positive discrimination by performing negative discrimination against another set of individuals.
If you notice that Zuckerberg rejected a person because she was a black woman, because she was black, and you counter that by somehow benefitting that woman, and having Zuckerberg pay some sort of penalty, then that is just and you have 'counteracted' Zuckerberg's negative discrimination.
But if, after noticing that Zuckerberg rejected a person because she was a black woman, you penalize him and make him hire some another black woman over where a white woman would have been more qualified, then you have not 'counteracted' anything. You have performed negative discrimination against another set of individuals, under the color of 'positive discrimination'.