The real reason is because of the needs of the plot, but if you want an in-story reason it comes down to how their powers work. Elune's power is overcharging the body to breaking point and the body is the one who has to bear the burden of that. Sylvanas on the other hand has her powers on loan from another and her body isn't under as much strain. Plus, said powers are from a type of magic specifically based on control, to the point where it's literally the name (domination magic). On top of this Sylvanas being undead has a more resilient body and one that doesn't have many of the needs of a living one.