All of the founders of the ACS were extremely rich men. No one really knows the exact reason why the society was started.
Most people believe that racism was the main reason.
In fact the purpose of the American Colonization Society and its 218 auxiliary societies was not to end slavery but to get free blacks out of the country. It would also be wrong to say that all members thought the same. Some members thought more on the line of abolitionist, they hoped that the society would eventually led to complete abolition of slavery. The idea behind colonization was simple. They just wanted to get all of the blacks out of the country.