How are you imagining the US government enforcing the abolition of slavery ca. 1800? Even in a much stronger relative position ca. 1865, it was extremely costly to do so. There was fair less abolitionist sentiment in earlier decades, and in relative terms, the federal government was far weaker and the southern elites far stronger. Attempting to outlaw slavery "quickly after the US started" (I'm assuming a window from about 1790-1810, please correct me if I mis-guestimated) would have been an act of suicide by the central government.