If you were being investigated by the FBI right now no one would know. If that investigation turned up nothing what would be the point of publishing it?
In this instance, everyone knows the Trump campaign is under investigation. If the report is buried there will always be conspiracy theorists speculating what's in it and frothing up the stupids. By publishing it the public will know once and for all if their president is innocent, a criminal or a traitor. You have a right to know. Trump has claimed he's innocent all along so let's let the record show that if that's the case. It's the best path forward to try to bring the country back together.
If it were me, and everyone knew I was under investigation but I knew I was innocent, I'd want the report made public. What am I saying, of course I'd be innocent.