Why does king say that America has defaulted?

In his "I Have a Dream" speech, Martin Luther King Jr. asserts that America has defaulted on its promise of equality and justice for all citizens, particularly African Americans. He likens this betrayal to a promissory note that the nation has failed to honor, highlighting the ongoing discrimination, poverty, and lack of civil rights faced by Black Americans. King emphasizes that the nation's founding ideals of freedom and justice remain unfulfilled, calling for a collective awakening to demand true equality.