They literally let the clock run out.
https://www.lawfareblog.com/where-justice-department-trump-obstruction-offenses
Where Is the Justice Department on the Trump Obstruction Offenses?
Exactly five years ago, on Feb. 14, 2017, Donald Trump pulled FBI Director James Comey into the Oval Office and suggested he abandon the bureaus ongoing investigation into Trumps former national security adviser, Michael Flynn. That small request was, in its own way, momentous: It was an early link in the chain of events that led to Comeys firing and the appointment of Special Counsel Robert Mueller.
Trumps hint to Comey that day appears in the Mueller report as the first instance of potential obstruction of justice committed by the president.
And as of today, Feb. 14, 2022, the Justice Department will no longer have the option of prosecuting that case. The statute of limitations for the various federal obstruction of justice statutes is five years. Starting today, assuming no indictment materializes between now and then, it will preclude the department from bringing a case against Trump for asking Comey to back off of the Flynn matter. As the clock keeps ticking down through the summer of 2022 and the winter of 2023 and eventually runs down entirely in May 2025, the possibilities for prosecuting Trump on obstruction charges for other incidents detailed in the Mueller report will dwindle further.
The heat map belowupdated from an earlier version published on Lawfare after the release of the Mueller reportshows how the special counsel evaluated the strength of the three common components of the obstruction statutes in the episodes he reviewed. These components include an obstructive act, a nexus between the act and an ongoing investigation, and corrupt intent. The columns on the right show when the statutes of limitations expire for the conduct identified by Mueller. The pardons granted by Trump to Michael Flynn, Paul Manafort, and Roger Stone in 2020, which could constitute an additional obstructive act, push the expiration date on the statute of limitations for those instances of obstruction back to 2025.
More at the link.
They go on to speculate about why, oh why, the dear DOJ didn't do a fucking thing.
Mueller (remember the breathless frenzied anticipation of his exhaustive sure fire take down of the criminal enterprise known as the trump oligarchy?) laid out the case and evidence. It was a slam dunk ready for trial case.
And poof.
Nothing.
Why?
Because they are either incompetent or corrupt.
And I don't think they are incompetent.
As George Carlin said "it's a big club and you ain't in it."