But when a Pope steps down, he's still able to glare at the cardinals who will choose his successor. He still holds them in thrall. In such an environment, nothing can change. Even if the pope is not in the room when the cardinals make their choice -- they will have to face him afterward.
I'm sure the pope understands this completely. It's the reason he resigned: so he can be there to manipulate the process of choosing the next pope. If he died "in office", he'd lose that control -- and this cagey pope is all about control. Above all, he wants to know that his "legacy" of hatred will continue into the next papacy.
What an awful creature he is.