THE SOUTHERN DEMOCRATS WANTED TO BE ABLE TO CAST VOTES FOR ALL THEIR SLAVES Without letting the slave actually vote himself.
This compromise allowed the north to counteract at least 2/5 th’s of that.
While I’m not about to defend slavery, women and children couldn’t vote either, and they were included for apportionment purposes, so expecting slaves to fully “count” isn’t quite as insidious as it sounds from the perspective of now.