Because, in general, there wasn't any popular vote.

The Constitution written in 1788 allowed (and still allows) Presidential electors to be chosen however the state legislature wanted. Some states, such as Pennsylvania (which had a high degree of populist sentiment) quickly adopted the system all states use today - the people would choose the electors from different tickets of electors pledged to vote for a specific Presidential candidate. Other states, especially in the South, had the state legislature choose electors. So, in (say) South Carolina in 1800, there wasn't any popular vote for President.

In fact, South Carolina was the last state to switch to popular vote, and that didn't happen until the Civil War. In 1860, every other state chose electors by popular vote - but in South Carolina, it was the state legislature. Electors were chosen by a state legislature only one more time: in Colorado in 1876, because the state had only been admitted to the Union a couple months before the Electoral College met, without enough time to set up an election.