You can read the decision and a press release. In short, the NLRB declined to exercise jurisdiction because it "would not promote labor stability." Why? Because:
By statute the Board does not have jurisdiction over state-run colleges and universities, which constitute 108 of the roughly 125 FBS teams. In addition, every school in the Big Ten, except Northwestern, is a state-run institution.The point being that most college football teams exist at schools that are governed by some kind of state labor act (like a "PERA" - Public Employee Relations Act). So, you would have different state labor relations boards ruling on the issue, and inevitably different teams would be governed by different interpretations.
I did not see this coming. Frankly, I'm not even sure I realized it was a possibility. In any event, it was a unanimous decision that managed to preserve the status quo in NCAA football while ducking a controversial issue.