Why did Northerners favor Wilmot Proviso and Southerners did not?

Northerners favored it because it banned slavery in the land obtained from Mexico, aka the Mexican Cession, thus making another step toward abolishing slavery. They supported this so much, that antislavery northerners created a new party, known as the Free- Soil Party, which supported the Wilmot Proviso. Southerners, however, opposed it, and wanted more land that allows slavery.