Why Ken Starr got it right and Mueller got it wrong.
Mind you, Starr will go down in history as a pervy Inspector Javert. Still...
Imagine you're an investigator in a local DA's office and you're looking into a local landlord who may be guilty of illegally converting single-family homes into multifamily rentals. So you're looking into local landlord dude and you run across extensive evidence that he is running a really big human trafficking operation. So, what do you do? Report back to your boss and say, "Y'know, I looked into the zoning thing and I really didn't find anything that looked actionable", while keeping the trafficking thing under your hat because you weren't tasked for it.
Hell no, you don't. You get that info out to somebody who can do something about it, whoever that might be. You're law enforcement, not a bean counter. It's your job to uphold the damn law. After investigating Trump for two years Mueller knows exactly who he is, with that knowledge backed up by reams of concrete evidence. If he can't find some way to get info out about actionable things Trump has done, it's because he has chosen not to.
Really, really makes me wonder.