Jefferson included anti-slavery verbiage but it was removed by the delegates.
The same situation applied to the drafting of the Constitution (BTW, Jefferson did not draft the Constitution, he was in France being an ambassador, but he did keep abreast of the drafting with steady communication with James Madison).
Delegates viewed slavery as something that they were unwilling to sacrifice the "American Experiment" over so they didn't. They viewed it as more important to set up and stabilize the new country and then perfect it. They did the best they could (always keeping an eye towards ratification which was contentious enough as it was) by including some ambiguous language in Article 1, Section 9:
The Migration or Importation of such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, but a Tax or duty may be imposed on such Importation, not exceeding ten dollars for each Person.
While this section does not specifically say "slaves", it was interpreted in such a way that Jefferson called for (in 1806) and signed (in 1807) the Prohibiting Importation of Slaves Act which went into effect on the 1st possible date (January 1, 1808).
Unfortunately, this law did not eliminate slavery as the slave population was at such a level that it was self-sustaining and imported slaves were not necessary to maintain the slave population.
You ask a good question and you answer it well: who knows? An historian can only address what actually happened and not speculate on what might have been. We only have to accept what has happened and live with its implications and repercussions. Eventually, enough time passes where folks have to just let it go and move on. One cannot undo what happened 150 to to 200+ years ago. Just apply the lessons learned.
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