Why wasn't slavery outlawed quickly after the US started? I would expect the free non-slaveholders would vote against slavery, since they wouldn't want to compete with slaves, and they'd outnumber the slaveholders.
In addition to the other points made in response to your question, a national law abolishing slavery would have needed to pass the US Senate, where each state got two votes, regardless of population. By the time abolition was something that might plausibly have passed the popular vote, the Southern states had formed a unified bloc on the issue. Admission of new states into the Union was explicitly evaluated on the basis of the balance of the Senate until this principle came into too much conflict with the principle of popular sovereignty.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.