Legally to be charged with murder you have to go in with the intent to kill. The intention to kill is crucial it is the heart of a murder charge. In this to find a guilty verdict the jury would have to believe he intended to kill her - from a legal perspective this is hard to prove in a case like this, intend to hurt/punish yes to kill no.
So correctly from a legal perspective (and it is a legal one) the charge of manslaughter with gross negligence makes much more sense and is far easier to prove (hence why he ended up having to plead guilty). This has the following:
the defendant owed a duty to the deceased to take care;
the defendant breached this duty;
the breach caused the death of the deceased; and
the defendant's negligence was gross, that is, it showed such a disregard for the life and safety of others as to amount to a crime and deserve punishment.
All of which he clearly was guilty of.
So it now comes down to the fact that it is seen a being a lessor charge - this doesnt have to be the case in the article the judge himself is quoted as saying:
'Whilst is it not under the charge of murder it is still an exceptionally serious offence.
*'I have ordered for a pre-sentence report.
'The fact that I am granting you bail, you should be prepared for a custodial sentence of some length.
'I will await to have heard from both sides and then I will decide on the sentence. All sentencing options are open.'*
The maximum sentence he can give is life - and indeed recent sentencing guidelines have been set out with a recommendation that the for serious culprits the sentence should be 18 years before parole
So legally the judge is correct. There are no mitigating circumstances other than the fact that the intent is difficult to prove in this situation. Judgement should really be held until the sentencing is given. If the judge (as he has indicated) gives a life sentence with a certain amount served before parole (around 10-18 years) that is not that different to if he had been convicted of murder.