Because at that moment, there was this superpower known as “England” that was bitter about losing them, and was wanting to kill them for treason. The military power imbalance was insane and they soon returned. The colonies had to come to a fast and dirty agreement that didn’t really please either of them fully to even have the slightest chance of survival.
The South got reduced representation, and the northerners that didn’t want slavery had to tolerate it a bit longer....to survive.
And there was an elegance to it. The south claimed slaved were not really fully human, and the north said, “Ok cool, does 3/5ths of human sound about right then?”. They used their own philosophy against them.
Had they not come to an agreement, had the South refused to join the union over insisting slaves counted fully, had the north refused to join the union over insisting slavery end immediately, the aforementioned England would have torn them to shreds, one by one.
That’s why.
Bkmk