It's because the good old USA won't hold run-off elections. If no one gets a majority, we should just hold a run-off primary (or election) between the two top vote-getters.
It's a bit expensive, but that way, the winner would have a true mandate from the people. And in an increasingly divided country, our leaders really need that legitimacy.
Why not have some form of instant runoff voting? There have been local elections with ballots where you rank the candidates, or put in a 2nd and 3rd choice right on the ballot. Then it’s just a matter of recounting the 2nd choices of anybody who didn’t vote for one of the top two vote-getters.
This ought to happen on the presidential level for sure to ensure the legitimate will of the people was respected. But either way such a change would have to implemented on a state-by-state level, and we see how all the various states already can’t agree on caucus, primary, winner-take-all, proportional, what date to hold their elections, even when the RNC penalizes them for doing it the wrong way.