Baz, the legal reason it was not ever going to be manslaughter is the reason the offence of "death dangerous" was invented. Juries refused to convict of manslaughter, even though it is manslaughter. They took the view that it was the kind of thing that they themselves, or people like them would do, whereas manslaughter was what criminal types did, so they wouldn't convict. And "death- careless " seems to have been invented later to satisfy the public, and the family of the deceased, who got very upset when a jury didn't agree that the driving was dangerous yet it had killed someone.
Manslaughter in driving cases is reserved for such deliberate or extreme actions as to be as near as dammit to murder
As to this case, the CPS, assuming they didn't have death dangerous on the indictment and took a plea to the alternative of death careless, must have thought it always just fell short of dangerous. You can get 5 years jail for death careless anyway. They might have had death dangerous there but thought it was not so winnable that if a plea to the lesser offence was offered, they'd take it, saving a trial. It's a bit academic whether it's opened as a low death dangerous or a high death careless because the judge will pass the same sentence.