The other prosecutions looked like non-starters from day 1, but I am extremely surprised that the driver was acquitted.
When multiple other prosecutions look like non-starters and the cases fall apart, people start to become skeptical of the whole incident. Unfortunate, but this is why prosecutors can't be allowed to go nuts and cater to the public. Every case needs to go through a carefully selected Grand Jury.
If a Prosecutor goes forth with an indictment because of public pressure to produce an indictment, or because they, personally desire an indictment, but they know, going forward, that they will not be able to prove their case beyond a reasonable doubt, the Governor of a State should (if he/she can) remove that Prosecutor from office, and the Bar Association of that state should investigate for the purpose of initiating discipline, up to and including disbarment.