J.J. Cooper of Baseball America, the publication that dreamed up the Futures Game, noted in a chat that organizations are limited to two players on the initial rosters, so the selection of Buxton and Sano foreclosed on Eddie Rosario and other Twins prospects. Cooper:
Overall, I think the Twins are bringing the best one-two punch of any team, and really it’s not all that close. ... Rosario would have been a very good fit at second base, but I’m glad he’s not there. Nothing against Rosario, but only two Twins can go and I can’t wait to see Buxton and Sano.
(Cooper's chat also touches on Adam Brett Walker as an example of why it's a good idea for teams to be hesitant about quick promotions, particularly in the lower levels; what Buxton's "80" speed tool means; and Sano's defense.)
In recent years the Twins appeared reluctant to allow more than one of their prospects play in the Futures game. Last year only Oswaldo Arcia played, and a BA chatter noted, without being specific, that they were turned down on some players by organizations. Sano came to mind; as noted in that link, the Twins view such things as rewards, and they probably weren't interested in rewarding Sano at that point.
This year is another matter. And by this time next year, Sano may not be eligible for the Futures Game.