A lot of it is regarding playing with Barzal. There were times where he played away from Barzal and did okay. Then there were other times where he played away from Barzal and was completely invisible. IIRC, Gropp's final WHL season, Barzal got hurt in the winter, and for close to a month Gropp was worthless. Then Barzal came back and they played together primarily, and Gropp's season went from terrible to respectable as he caught fire.
Then there's the fact that a lot of people here wanted guys like Sprong, Rasmus Andersson and Bracco before the draft and definitely at that pick. Now, none of those guys have established themselves as NHL players, but they've all done more to impress than Gropp so far (maybe not Bracco, I think he's pretty meh). But, as we know, when the board latches onto a guy or guys and they're not picked, if the guy we did pick isn't as good the pitchforks come out--as if the team has never made a better pick than what the board wanted, but whatever.
I think the idea persists that he'll only be able to score with a really good center. That might be true. It might not; he could develop a better game away from the puck and turn into something more. I just think that regardless of all those other things, he still has the skills to warrant a spot here. And, he's still young. 22 for the season. He could spend another couple years developing and then make it at 24 and it would be pretty normal.
Personally, I think the Barzal thing is a bit misleading and was even commented on by a poster or two who focuse on Seattle.
Over a four year span, Barzal played in 202 out of a possible 288 games with Seattle. Some of it was because of injuries, some of it was international play, etc. etc. But the fact remains that Seattle played roughly 30 percent of their games without him, or roughly the equivalent of a shade under 1.25 seasons.
That's a lot of hockey.
Unfortunately, without Barzal, Seattle's depth, particularly at center, was pretty thin.
Aside from the misconception that Barzal and Gropp were joined at the hip when Barzal was in the lineup (they weren't), I don't know if the board has ever fully understood what Gropp did (or didn't) have to work with in that lineup. Not that any of that changes what he needed to work on, but I never quite felt there was a levity there.
Factor in the board preferences that you mentioned, and the fact that he was included in a trade that didn't work out for the Rangers and saw a pretty popular player traded, and there's definitely a recipe for a scenario in which everything this kid's done, or hasn't done, is magnified.
None of that takes away from his disappointing rookie season, but I don't know if it was quite as bad as it's been made out to be either. And that's probably the bigger issue with Gropp --- there's always a bit of an undercurrent of borderline rooting against his success. Unfortunately, that's not exactly unheard of around here.