Because a 1D >> 1W, then the winger has to out produce the D by a significant amount or have a much bigger impact on the game.
Marner ----- 77 games 19 goals 42 assists 61 points
Werenski -- 78 games 11 goals 36 assists 47 points.
A 47 point rookie Dman >> 61 point rookie winger and the Calder voting backs that up, as playing D in the NHL at 19 and excelling much more difficult/impressive than a sheltered winger position.
Also finding scoring winger are much easier to obtain, as JVR (29g 63 points) and Nylander (22g 61 points) also scored more and provided similar points to Mitch.
In addition 3 other forwards Matthews (40), Kadri(32) and Brown (20) scored more goals then Marner and the 1st X 2 also more points.
Whereas Werenski 44 g & 47 points > Gardiner 9g & 43 points (Leafs top scoring Dman), would have lead Leafs entire D in scoring.
In the laws of supply and demand it demonstrates on team needs the value of a top pairing scoring Dman verses a scoring winger who is 6th in team goals and 5th in points.