Why? Stereotypes about black intelligence.
There is a growing body of research on the effect of stereotypes on test-taking -- called the "stereotype threat". Blacks, when given an exam that tests their intelligence, consistently score lower than whites. But when told that the exam they are taking is simply practice, blacks score on par with whites.
The key is distraction. If given a test that explicitly measures their intelligence, blacks will become distracted by the those things that others says about their intelligence. Even if a black person is absolutely positive that the black intelligence stereotype is, in fact, myth, then that person is likely to be distracted during the test by the thoughts of the stereotype in their head. If a test is time sensitive -- like most SAT or GRE exams are -- then a couple minutes of accumulated distraction can make a significant difference in scores. Now, replicate a stereotype threat across many years of schooling and many tests; a student is likely to suffer.
Of course, this is not limited to black stereotypes. The same results have been found with women on math tests -- it is "common knowledge" that women are not as proficient in math as men -- and whites on exams when compared to Asians. In all cases, the gap disappeared when the stereotype was put to rest.
This lends new evidence to restraining oneself from the use of stereotypes. If the fact that these hasty generalizations are false does not dissuade you, then maybe the knowledge that the proliferation of the stereotype significantly and negatively altering the lives of others will convince you. As long as a stereotype is strong enough in society to enter the minds of the target group during critical moments -- be it an exam or an interview for a job -- the affected people will continue to suffer, individually and as a group.
It is a self-fulfilling prophecy. If enough people believe the rubbish we consider "common knowledge" about different groups of people, then that rubbish will find a way to, at least in part, be expressed at important moments.
So in using stereotypes, remember the power you wield. Ask yourself if you think the generalization is really true. Moreover, ask yourself if you want it to be true.