Why did Abraham Lincoln make it illegal to have slaves in the US?

The first slave arrived in 1609 with the start of colonies, but the issue of slavery was always around. The northeastern states outlawed the import of slaves by 1640, but a child born into slavery was a slave. With the invention of the cotton gin the need for slaves grew in the plantation system through the 1860's. With the civil war and the passage of the 13th, 14th, and 15th amendments of the constitution slavery was finally outlawed in 1867. The import of slaves was mainly an economic issue and it was cheaper to not import than to use the slaves that were born into the system.