Probably because the characters need to feel a match. The readers need to want and believe they belong together.
When you already have characters linked as a couple the acceptance is automatic. Anything between them is what we either expect as couples, having experienced it ourselves. An arguing or patterns are seen as natural – how these two always are.
And it’s natural to break them up, if needed, because we know they have a history even when we don’t know the history. We were not invested in their courtship. We didn’t see what attracted them to each other.
When writing a romance between new characters we need to figure out why they fit. Why we want them together. What brings them together? Do they match what each are drawn to? Yup, I’m talking as if they’re real. Because they need to reach your reader as being real. And given all our backgrounds direct us to the person who we should be, want to be with. And if it’s an opposite or someone from left field – why?
Then there’s the whole part of how they meet. What did they have in common that brings them to each other? If you’re writing a straight-up romance – no other genre – then you really need something reasonable.
Yeah, this isn’t an easy genre.