Forcing someone to vote for only one person or idea in an election forces them to ignore options they might actually like. Even worse, it stifles competition, because competition splits a big group of votes for one person or idea into a smaller group of votes for multiple people or ideas.
Instead of forcing voters to pick one option in an election, letting voters pick multiple options fixes a lot of problems. If voters can pick multiple people, they can always pick their favorite person, and can pick someone else as a backup. It also means that new people and new ideas can enter an election, because voting for new people and new ideas doesn’t automatically take away votes from the people and ideas that are already there.