Because all that needed to be proven was reasonable doubt.
Writes unsigned editorial writer:
The other two conviction options available to the jury – second-degree murder and voluntary manslaughter – would have required the jury to find that Mehserle meant to kill Grant. The evidence indicated the officer’s state of mind was contradictory at best. His reaction immediately after the shooting suggested disbelief at what he had done. Yet his explanation of having mistaken his gun for a Taser did not emerge for several days.
In other words, there was reasonable doubt about his intent, which was the standard the jury needed to overcome, even if that will not fly in the court of public opinion.