Feels wrong.
Another could be concern of doing anything visibly election related prior to the election, in case the winner didn’t like the action.
Of course, the exact meaning of "necessary" is the problem. We can probably all agree that letting a serial murderer kill more people in order to have more evidence against him would be an absolute abomination. But the situation is not that clear with white-collar crime, especially the sort of crime where the harm is not immediately obvious.
Maybe the fact that the FBI could wait that long before pouncing indicates that this sort of betting is, in fact, a victimless crime.
That is simply not how it works and for good reason. If it were, imagine the consequences. Most charges would never lead to a trial or a conviction because law enforcement would have acted on the very first sign of criminal action instead of investigating further or working their way up the chain. We'd only end up arresting low level foot soldiers and the higher ups would get away every single time.