In 1789 the agricultural products of the slave states represented a very large chunk of the USA's GNP. If you wanted a USA that included everything south of Maryland, Virginia potentially being a toss up, any limitations on slavery were simply a n-go. It was a struggle to bargain the south down to the 2/3 clause as opposed to counting slaves as full "persons" for representation in the House (and of course for electors). If it is pushed too hard you could end up with two (or more) polities - one being New England plus NY, NJ, PA, DE the other being NC, SC, GA. Exactly which way Maryland & Virginia might go is a bit of a toss. I can Maryland going north and Virginia south. Further subdivisions are possible. Of course this makes the issue of land claims in the Ohio territory, and west of the Appalachians way more contentious as they are now disputes between nations and not states. Also, since the British were still occupying posts in the Old Northwest and aiding the Indians against the USA this gets even murkier.
Getting states in the north that had a residua of slavery to buy in to this would be easy, where it was a little stronger like Delaware and Maryland still doable. South of that, where slave produced products were a major money earner - no way. Insist on such a clause, or even not counting slaves at all for representation, and you don't get a constitution and the USA breaks up.