Good question. Because it will increase the probability of Electoral Votes going to third parties. That decreases the probability of any candidate reaching an outright majority of Electoral Votes. A plurality of Electoral Votes is not enough; it has to be a majority.
Without a majority, the election is decided by the House of Representatives, where each state delegation gets one vote, where each vote is determined by a majority of that state's delegation.
And there, the Republicans have a built-in advantage similar to their Senate advantage, as every state is guaranteed at least one representative regardless of population. Even after the Democratic House victory in 2018, Republicans still had the majority of 26 state federal delegations.
Without a majority, the election is decided by the House of Representatives, where each state delegation gets one vote, where each vote is determined by a majority of that state's delegation.
And there, the Republicans have a built-in advantage similar to their Senate advantage, as every state is guaranteed at least one representative regardless of population. Even after the Democratic House victory in 2018, Republicans still had the majority of 26 state federal delegations.