I think there's a few reasons..
Limited viewings/limited evaluation when viewing: He played in the Detroit area of course, so some teams may have almost solely relied on tournaments to view he and his teammates, which means they saw him a lot less than anyone playing in Ontario. Even when they saw him, he's an underager and you're paid to scout players that are eligible in the current season. You probably won't pick apart his game but rather jot down a few things since he's a year younger than everyone else. The talk of him looking at ES picked up at the Silver Stick, which at that point the MM season is basically already halfway over. That means little time for serious evaluation, especially when you consider the fact that he didn't play in the province.
Rawness: He's pretty much a blank canvas. He has some amazing tools that basically make his ceiling unlimited, but he needs to learn how to fully utilize them. For example, I believe he has had very little defensive coaching. Either this season or last season was the first one he had serious defensive coaching. This can be looked at as both a positive and a negative. If you look at it positively, he's already an incredible prospect and still has plenty of room for growth. If you look at it negatively, he may not be ready to step into your lineup without some growing pains. The Steelheads obviously looked at it in a positive manner and are willing to help him grow.
Conspiracy: Some think that Branch wanted a star in the GTA and made it happen. I don't really know about that and the other two factors make much more sense.
Character and reporting, as previously mentioned, should not be factors as he was granted ES.