Because really our only blue chip defenseman prospect is cal foote. By 2020 there is a strong chance he can make the nhl roster
Even under the assumption that Priskie doesn't mind going through the pipeline and not getting immediate opportunities to play(why would he want to do that), he's still behind Cernak and then there is Sergachev.
The only avenue I see for him here, is if he signs to replace Shattenkirk next year, and then we have him waiting in the wings to possibly replace Sergachev if Andrei is wanting major $$$ and we have to trade him.