Josh Marshall nails it (paywalled, but here’s part of his argument):
Published reports suggest – and it is no surprise that this is the case – that the DOJ investigations are not looking deeply into the causes of the January 6th insurrection, causes which are inherently political and tied to numerous public officials and electoral politics. They’re going to look at the specific people who broke into the Capitol. They’re going to look at organized groups that may have planned the acts of violence in advance. This may net various Proud Boys or Three Percenter groups. It may even net a particularly zany member of Congress. Those people deserve to face legal accountability for their actions.
But there will be much less scrutiny into why the whole thing happened, which directly implicates President Trump and numerous of his political supporters on Capitol Hill. It may not implicate them in crimes. But it implicates their responsibility. We don’t know just where the DOJ is going to draw the line on this. But even to the extent it takes a broad view of its brief everything it finds will remain secret unless and until the information becomes necessary to reveal in a criminal prosecution.
Which is to say most of it will never see the light of day. Ever.
Needless to say this leaves people like Mitch McConnell with a huge interest in saying that the DOJ has this covered and any other inquiry would be redundant and even compromise the DOJs investigations.