It was designed so that the slaver states got their 3/5's representation counted for Presidency as well without actually having to treat the slaves as humans and, god forbid, actually let them vote in a popular election. Even if it was simply a popular vote of white men, the southern slaver states knew that they'd be dominated politically, and so instead set up a system where they could more often than not dominate the majority of Americans.
When it was originally conceived, there was no conception of a broad popular vote or political parties coming to dominate who even gets consideration. For a while, a number of states didn't have a presidential vote at all and instead had the state legislature choose.