Quotas discriminate against people of different ethnic background who now see their chances for admission reduced regardless of qualifications. The “pro-quota” race-focused collectivists may think they have solved the discrimination against the race they are concerned about, but they have only shifted the discrimination to another race—they have “solved” alleged racism with more racism.
Substitute gender, class, age, disability, religion or some other group construct for race, and you can see how across society, group-against-group “rights” clashes are perpetuated and will escalate if not counteracted.
Think Right or Wrong, Not Left or Right 