The new American Congress outlawed the transatlantic African-slave trade almost simultaneously with the British Parliament, but we were stuck with the slaves we had.
Many of our Founding Fathers who owned slaves saw slavery as evil and some even tried to gradually end it. Charles Carroll of Carrollton for example, who signed the Declaration of Independence introduced a bill in the Maryland Senate to gradually abolish slavery (it didn’t pass). He is quoted saying, “Why keep alive the question of slavery? It is admitted by all to be a great evil.”