I am no expert, but as I understand it, the CPS needs to prove beyond a reasonable doubt that the death of the victim was the accused's premeditated intention, in order for a charge of murder to be brought, and for a weight of evidence to provide a better than fity per cent chance of a conviction.
In terms of manslaughter, as I understand it once again, a degree of intent has to be present, and in this case, it was not present, hence the absence of the charge.
Looking at a siutation as a simple scenario, and deciding that an accused should be accused of something that will generate the maximum sentence, is not the way the law works.