It becomes a legal proceeding. Like any legal proceeding, your trauma now has to be relived over the course of a criminal trial, with you having to go up on a stand and recount the entire experience and be called a liar in a public forum by opposing counsel and have all your neighbors (and the world) know more about you than you'd like and fashion all manner of horrible opinions of you.

Nobody wants to go through that.

Not to mention when your attacker is someone you know, and you wrestle with whether or not you might have implied consent will result in absolutely ruining someone else's life with the mere allegation.

It's not a light matter. Most cases actually aren't performed at gunpoint in an alley.