You were so erratically powerful and so slippery & therefore unchecked for so long that you contained a treasure trove of dark arts abilities that I myself would NEVER see fit to investigate myself in the first place (if for no other reason than the intolerability investigating such would cause me).
So in (death) I saw solution
But in keeping you (alive) I saw opportunity
So, not a case of couldn’t but instead wouldn’t
Aren’t you fortunate, Zetsu?