Unlike murder which is intentional, manslaughter is strictly unintentional.
Once it's determined that there is not enough evidence to prove murder and there is reason to believe that the killing is accidental, I think there are a few things to consider.
Firstly, from a purely economic standpoint, what is the benefit to the economy and to society of imprisoning a person who unintentionally killed someone and probably won't do it again?
Secondly, even if it should be removed from the criminal code, this still would not stop the relatives of the victim to sue for damages to them. And how is the perpetrator to pay if he's in jail? I honestly don't see how criminalizing the unintentional benefits anyone?
That said, I am open to allowing capital punishment for murder though due to the intent behind it.