Because buses were an explicit scenario of racial prejudice.
Explanation:
Blacks used buses with the same frequency as whites, in addition to paying the same fare. This should allow them to have the same right of use, since the requirements for using the buses were the same as the requirements for whites. However, blacks did not have the same rights of use, since they were forced to sit in the back and give up the place if there were not enough chairs empty for whites. This made the bus an explicit place of discrimination and that is why it is so important for protests in favor of civil rights.
