She wasn't found dead at the hands of her boyfriend. The evidence did not suggest, let alone prove, that the injuries he caused were life threatening. THAT was the reason why it wasn't murder.
Indeed. Her blood alcohol level on its own was in the bracket where coma and death would be expected. She had also taken cocaine, amphetamines and poppers. One of the experts said he had never seen such high levels of alcohol and cocaine together. The prosecution could not rule out the possibility that the death was caused by the alcohol and drugs alone. For a murder conviction the prosecution would have to prove that her injuries contributed to her death. They were unable to do so.
There was a further problem in that the prosecution also needed to prove that there was intent to kill. One might think that the injuries clearly demonstrated intent but there are problems with that.
Most of the injuries were bruises. There was clear evidence from prosecution witnesses, including the victim's sister, that she enjoyed rough sex and proudly displayed the bruises she received from previous sex sessions. That undermines the bruises as evidence of intent to kill.
There was a more serious injury caused by a bottle being inserted in the victim's vagina. This caused arterial bleeding, probably when he tried to remove it. This may be GBH but it was consistent with an accident (if it was done with her consent, as he claimed) or sexual assault (if she did not consent).
Finally there were the head injuries. These were widely reported in the press as being caused by punches, as alleged by the prosecution. The problem is that the medical evidence advanced by the prosecution was that the injuries were consistent with the victim stumbling around in an intoxicated state and injuring herself. There is no proof whatsoever that her partner inflicted these injuries.
So the prosecution could not prove that the injuries caused or contributed to her death, nor could they prove that there was any intent to kill or cause serious injury. On the evidence, it seems to me that there was never any realistic chance of them securing a murder conviction. Instead they accepted a plea of manslaughter due to gross negligence - he didn't call for medical assistance despite her obvious injuries and gross intoxication. On the evidence that appears to be the right verdict. I note that the victim's family were entirely content with the prosecution's decision to accept the manslaughter plea and drop the murder charge.