It's not a case of expecting him to make the NHL. It's that thus far, his impact in the NHL hasn't been anything of particular note, at least not for someone drafted 2nd overall. He was expected to take large and fast steps in his development this year. Having a good camp and making the NHL (even on a weak team) is of some note, but unless he starts making more of an impact, I don't see him as having shown enough to say he's exceeded the expectations. And seeing that it was the precisely the expectations of his development taking a jump this year that raised his stock so high that he was a 2nd overall pick, I don't think his stock is really raised by him actually taking a jump in his development.
And really, it's not like he was beating out a ton of quality depth to make the Pittsburgh roster. The only people on the team seeing less icetime than him are Chris Thorburn, Karl Stewart and Andre Roy. Beating them out for playing time isn't really a significant accomplishment unexpected from your usual 2nd round pick. Unless he manages to increase his playing time and have more of an impact, his making the NHL actually drops his stock a little in my eyes, as he won't be getting the development he should be in what should be an extremely key year in his development. However, I rather suspect he'll be returned to juniors before the 10 game mark anyway, so it's a bit of a moot point.