I think one of the things the Avs may have been impressed with was his awareness of the weaknesses in his game, and a strong willingness to improve on them to be as good as he can possibly be.
He mentioned in that interview he was 6'3" but thought he played as if he was 6'1" (clearly someone said this to him somewhere along the line) and that he wants to play more physically to take use of his size. Someone else mentioned before about him wanting to play more physically, and he's also acknowledged he could improve his skating.
These are the kinds of things teams love to hear, and why they do the interviews. It's very easy to downplay your weaknesses, or even give lip service to improving them, but when a guy sounds highly committed to improving them, especially physical play which many players don't talk about improving, it shows a lot of character and commitment to doing everything they can to help the team win.
Take Seth Jones for example. I'm not knocking him that much, because I wanted him as the Avs pick, but he all but said he would never change his passive style much when it came up in interviews. He'd always point to Nick Lidstrom as being successful without doing so, as if every defenseman can be one of the top five best defenseman in history by playing a more passive game.
From reports out there Jones' interview was average at best, and I'm not too surprised. He doesn't come off all that great. It doesn't seem to be a case of him being too serious, or trying to give standard boring answers ala Tavares. It's something else, and it's tough to know what to make of that. Is it an example of him being cocky, like a lot of people accused him of being when he pronounced his USA team would win early in the tournament? Maybe, maybe not. Maybe it's just an example of him being forced to overcompensate for being naturally shy or something.
Maybe they had some concerns over how coachable Jones would be, and whether he thought he knew how to play the game more than his coaches, and wouldn't let the team develop his overall game. Maybe they started to have too many unanswered questions, and saw how great Mac and Barkov played, and then saw how great they came off in interviews, and that put it over the top.
Just speculation on my part, but I think something happened somewhere along the line, and it wasn't just the Memorial Cup, or a preference to taking a forward over a D.