How does a suspect not get charged with raping/impregnating a minor?

First off, I am really sorry this happened. It's awful.

I can't tell you UK law but I can conjecture about American law (I am a retired lawyer but haven't done any criminal work since 1985).

I would guess that there was no good DNA evidence. If there was a baby born, has there been a paternity test? That would place the perpetrator there.

If there's no paternity test, and there was an abortion or a child was adopted, then it's harder to put the perp at the scene.

But all that goes to proof. It doesn't go to the question of whether the guy could be arrested and charged, both of which are lower standards of proof.

Plus she is still a minor. It is 100% proven that the pregnancy was the result of a male having sex with a minor.

I don't know what your statutory rape and consent laws are, but in a lot of the US, a difference of 4 years plus either partner being a minor equals statutory rape, regardless of consent.

If the guy wasn't convicted, then that's one thing. But to never charge him with the crime?

Either your girlfriend was 100% uncooperative (which can be due to trauma), there was no good DNA evidence, the cops are incompetent, and/or money changed hands.

I highly doubt that the Statute of Limitations has run (if there even is one). She could, potentially, have charges brought now, or sue him in civil court for battery. If there's a child who she's raising, she can push for a paternity test and for child support.

But this all requires her cooperation, her mental energy, and her willingness to go forward.

Note: I am a lawyer, but I am not your lawyer. If you or she want to see this through, then consult a solicitor who specializes in underage rape cases.