Hellebuyck, Gibson and Demko are certainly names that you can't thumb your nose at, but with Campbell quickly reaching elite status, it is a bit surprising that he was not called upon to represent his country - specifically because he leads all American-born goalies in every major statistic this season.
The primary reason that Campbell did not make the cut is likely due him not having a lengthy history as a starting goalie, nor does he have a ton of NHL games under his belt. In fact, Campbell has played just 109 career NHL games.
If that's the argument that you wish to make, it may surprise you to learn that Demko has started just 96 games in his NHL career. Granted, he's almost 4 years younger than Campbell, but his career numbers are nowhere near as good as Campbell's either.
With Hellebuyck likely coming in as the starter and Gibson likely coming in as the backup, I suppose we shouldn't be splitting hairs over which of Campbell or Demko should land in that 3 spot, but it just seems odd that one of the league's best American goalies over the last two seasons is on the outside looking in.