He created a site where you can upload anything with complete privacy and anonymity. And then used it for racketeering, allegedly, which is where the government interest starts. The RIAA/MPAA want their pound of flesh, too, and it gave plenty of fertile ground for the US DOJ to build a case around so that they could get discovery and find out what they were really trying to get access to. But the piracy is not the point; not by a long shot.
As with anything that allows absolute anonymity AND absolute privacy, it's bound to attract bad actors. Yes, the "pirate music" types. But ALSO the "sell humans" and "provide criminal services (hitman/fraud agent/patsy agreement/etc)" types.
Dotcom can turn blind eyes all he wants, but if won't take responsibility for the damage he is facilitating, it is in the public interest for him to be held accountable against his will.
I'll never stop pirating media, and I'd never want a media pirate to go to jail. But I'll never defend a human trafficker either, no matter how "innocent" they allow themselves to remain via intentional ignorance.