It's familiar and trying to get 4 people romantically involved is a lot harder. That's why polyamory stories hardly works unless the author goes through a harem route where there's a focus character tying them all up. But I've seen harem stories where the other girls diverge from the MC and create their own character than being a side character which the MC can choose anytime, the problem with this route is how it's harder to write about since the focus of the story is harem romance. Not saying the love triangle trope is bad nor is it good, it's just familiar and easy to produce as a base for a story.