Timing. It's likely a gold mine of bad info on bad players, so sit on it until you can reap the most benefit. Rock the midterms with a bomb 6 weeks before election day, take the heat off Kavanaugh etc.
Yes, Mueller is going to find collusion. Unfortunately for the democrats, it's going to be between HRC and the Russians. Obama's FBI was against Pres Trump as a candidate and all that they did and tried to do is going to come out. I'll bet there are more trials. McCabe, Comey Page...to name a few.