So why have we never heard of them?
Because Colvin was a knocked-up teenager and Smith's dad was rumored to have a drinking problem. Rosa Parks, on the other hand, looked like a 19th-century school-marm:
Months before Parks refused to give up her seat, local members of the NAACP were looking for the right test case: someone who could be the face of the civil rights movement as they took their cause to court. Colvin didn't work because not only was she was an unwed, pregnant teenager, but the father of her baby was an older, married man. Plus, she was described as "mouthy" and "feisty," which didn't sit well with leaders who knew that whoever gained notoriety would be a target for every racist on Earth. Likewise, Smith didn't work because word on the street was that her dad was an alcoholic, an allegation she later said wasn't true at all. She didn't even find out she had been in contention for the role until a reporter let it slip in 1995.
Parks, on the other hand, was perfect. Colvin's own mother cited Parks', um, lighter skin color and likable personality as reasons why she should be the one.
It sounds cynical, but you can't argue with the results.