Because Reinhart in Juniors was the straw the stirred the drink. Then they saw Eichel really get in a grove when he was put with Reinhart and both guys seemed to find that chemistry. Add in the chemistry that Reinhart had with O'Reilly and people figured Reinhart was the cause of this, so they had this idea that Reinhart would be okay since he was the common denominator for everything that was going so well.
Now since we have a chance to see him at center, due to Eichel's injury, people thought, hey, he can play center, he's shown he knows where to be and those type of players usually play center. Only problem is that Reinhart isn't paired with the smartest hockey players in Girgs(which his intelligence has been debated in the past) and Ennis. Reinhart works well and can read the play better when paired with someone who thinks the game at a high level as he can.
Add in the fact he was a 2nd overall pick, and there's the recipe for what we are seeing from our fans. They expect him to impact the game like past 2nd overall picks have in the past, when in reality that was NEVER his game in the first place.
Where he was picked really amps up the worrisome posters.
