Because he doesn't really have the puck skills, passing, or shot to play any other role. But, what would make him good at the net front role? His ability to play with his stick upside down?
I get that just about every players numbers would be better if given a bunch of PP time. That doesn't mean that every player should be given a bunch of PP time. Some guys are just not likely to ever be anything more than very good 3rd liners. I think the Wild have 3 of those guys right now. I think all 3 of Greenway, Ek, and Kunin can probably give you quality minutes on a top 6 line without really hurting your team, but none will ever more valuable in the top 6 as they would be as shutdown 3rd liners. Kunin could be an exception if he ever learns how to catch a pass.
Playing a guy like Ek between guys like Zuccarello, Kaprizov, or Fiala would be great in their own end. It would probably lead to spending quite a bit more time on offense. The only problem is that the amount of plays that would die because of Ek's lack of skill would more than offset any defense that he plays.