The Hart Memorial Trophy is awarded to the "player judged most valuable to his team". With the Rangers, yes Shesterkin is important, but there are a number of reasons why they are where they are right now.
Chris Kreider having scored 34 goals this season is an obvious one. There's just nothing in Kreider's scoring history to indicate that he will be able to keep up this type of production and, when the well dries up, the Rangers will be relying on someone else to pick things up from an offensive perspective.