I don't know about Galchenyuk (Bergevin not optimizing him may have played a part) but Miller scored 22 goals while not on the Rangers' first line, was a positive Corsi rel player the past season (unlike Jenner), and is a winger full-time, which I suspect is the most important, considering the number of centers already on the roster. It means they're not converting yet another player to wing.

Furthermore, he won gold on the 2013 WJC team with several other members of Team NA, which could have been a consideration as well. Though obviously so was Galchenyuk.