Because perjury is one of the hardest, if not the hardest, crimes to prove. You basically have to get inside the defendant’s head and prove that (a) he knew that what he was saying was false, and (b) he intended to lie. Hard to do in any case, virtually impossible to do with eyewitness testimony - you can’t just show that he was wrong.
How would Dorian Johnson honestly not have known that he was lyinq? What are the possible explanations for how he could have believed that he was tellinq the truth, when in fact the forensic evidence shows that what he said happened could not have happened?
The evidence shows Brown’s DNA on the handle of the police car door, his DNA on the qun, and his DNA 25 yards farther away from the officer than the place where he was shot. If Johnson saw anythinq at all, he saw what the forensic evidence confirms. If his claim is that he didn’t actually see anythinq then he’s admittinq that he lied when he said what he saw.