Why did progressives and dixiecrats boltthe Democratic Party in 1948?

During the 1948 presidential election, southern Democrats, known as 'Dixiecrats,' chose to leave the party because of its civil rights policy. It had evolved from its earlier pro-slavery leanings to become more race friendly, and the Dixiecrats didn't like that. They wanted to start a new party that focused on states rights. The Progressives didn't bolt from the party, in fact, they were a separate party, when formed in 1946. The would eventually merge with the Democratic party.