Yes, Tito was a war criminal and an unprosecuted one. However, Tito's murders were just that -- political murders.
But the Ustashi killed primarily based on religious & racial identity -- Serbs, Jews, Gypsies. Not just murder, but what we would today call a hate crime, religious persecution, literal "ethnic cleansing" -- and for the most part, they got away with it. But, better to go after them late than never.
Why go after WW2 era when there are plenty sitting pretty in Zagred, Sarajevo, Tuzla, Hrvatksa and throughout Kosovo. Forget "WW2" era, no one cares about the past, get the current era whilst it is still freshly engrained into the current generations head. They'll remember the arrests and repercussions.