That's the thing. If they were struggling to find evidence, they would want to get them on the first thing they could and stop their corrupt tank from rolling on further.

But.. if they had them stone cold, with all the evidence they could need, an abundance of evidence, there is no harm at all in letting them continue their scam, building up more and more against themselves, and elevating themselves in every possible way before the public. You can pull the plug at any time, and their machine grinds to a halt. If you are in a comfortable situation of having overwhelming evidence, you really can engineer a reveal that will have the absolute maximum impact for generations... for centuries...