Size, and lack of physicality. Even Bedard, with his unworldly talent, has had difficulties, and players like Rossi are getting moved because of the perceived disadvantage that their lack of size and strength gives them in the NHL. Kent Johnson had a lot of trouble early on, and it was only after a few years and a rigorous summer of training, as well as moving to wing, that he got right. i believe Johnson is bigger than Hagens, too.
Guys like Byfield, Lindstrom, and Martone will always be preferred over small perimeter players if the talent is similar.