Racial & ethnic groups

In the 1950s, “people of color” in Montgomery, AL, were required to sit in the back of buses, reserving the front of the buses for Whites only. If buses were full, they were then required to give up their seats to White passengers. Rosa Parks famously refused to give up her seat & was arrested for that offence. This eventually resulted in changes to the law. Why did it take such a startling example to change the law?