The stereotype threat effect is worse for those who try the hardest because they are the ones who care the most.
Stereotype threat occurs among people who belong to a group that has negative stereotypes held about them. When such a person is in a situation where they can prove or disprove the stereotype, they tend to become anxious and perform worse than they otherwise would on the given task.
This has the most impact on those who try hardest because they really care. They know that they are capable and they want to do their best so they can show that the stereotypes about their kind of people are wrong. Because of this, they put added pressure on themselves and fall victim to stereotype threat.