Some have opined that Mueller turned it over to his Congressional Democrat pals once they were in position to harass Trump with demands for tax returns, testimony from every aide, etc. The OSC was both a “perjury trap” and an “obstruction trap”; when both failed, it was time for Congressional showtrials and a pseudo-impeachment process.
The Dems know they have no basis and can’t succeed, but that’s not the objective. Like the Mueller investigation, it’s intended to: harass, aggravate, and distract Trump; hope Trump will do or say something in response that creates an opening for them to exploit; undermine Trump’s ability to work with Republicans, by giving them leverage over him (Congressional investigations mean he needs allies; friends don’t come cheap).
This is what populates the Deep State, the US Congress, and the news media.
Washington is about power. There are no friends. As Harry Truman said: "If you need a friend in Washington, get a dog."
When an honest man thrusts himself into their midst and demands TRUTH and transparency, his very presence threatens everything that such people hold dear--their power, their delusions, their ambitions, their wealth, their cherished positions of success, their deceptions, their fictions, their beliefs, their self-concepts--their entire worldview.
The very intrusion of such a man incites terror and fury, as TRUTH always does when it threatens delusion, and they imagine him to be false, insane, stupid, power-hungry, and everything that they themselves are but struggle to deny; they correctly perceive him as dangerous; and, from their viewpoint, they MUST destroy him.
If that man is of towering intelligence, puissance, wisdom, and prowess, they panic. Their situation is desperate. They cannot understand what's happening. They cannot believe that they, having reached what they consider the pinnacle of power and success, could not be the wisest, most intelligent, most puissant, and probably the most benevolent people ever created, and, therefore, that anyone who opposes or disagrees with them must be the opposite of all that. They can neither accept nor defuse. They are lost in confusion. They are dangerous.