It is easier to cheat via popular vote than cheat by electoral college. Cheating appeals to the Democrats. Republicans prefer a fair game.
Cheating is very easy with the popular vote. For example, each state has a roll of registered voters. On the average, only about 60% of these eligible voters will vote in the presidential election. All you need to do to cheat is vote for the 40%, who are eligible, but who did not vote. This is what California did in 2016. This is easy yo do, using computers. You compare the eligible voter rolls; 100%, with actual voters; 60%, and then add votes from the 40%, who did not vote, blended into the late vote count. It will look legitimate on paper.
With the electoral college, popular vote cheating, using this and other techniques, has no affect, after any candidate gets a majority. Whether your candidate gets 51% or 99%, voter cheating gets the same electoral votes. This is what happened in 2016 to California. They voted at over 70%, which is unheard of. This did not get them any extra electoral votes. California would not allow their voter rolls to be inspected for cheating.