Zimmerman was acquitted because he committed no crime and an independent witness testified that Martin was slamming Zimmerman's head on the concrete and raining down blows on him "MMA style". As for Blake,
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After 12 weeks, Robert Blake's trial ended with his acquittal, as reported by the Los Angeles Times. The jury deliberated for nine days before coming to a conclusion. According to the jurors, they did not believe the statements of the two stuntmen who testified that they were solicited for murder. The defense was able to poke holes in the witnesses' credibility, as they both admitted to using drugs that could have caused delusions.
On the charge of murder, on the other hand, Blake was acquitted because there was no forensic evidence that could prove that he was the one who fired the gun that killed Bonny Lee Bakley. Furthermore, there were no witnesses who could say that Blake was near his vehicle when Bakley was shot. Overall, the reason why Blake was acquitted was that the prosecution wasn't able to prove — beyond a shadow of a doubt — that he paid someone to murder his wife, nor that he had done the deed himself.
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The Real Reason Robert Blake Was Acquitted Of His Wife's Murder - Grunge"
the prosecution had no witnesses, no physical evidence and couldn't make its case beyond a reasonable doubt.
In OJ's case there was plenty of evidence, a witness that OJ wasn't at home when the limo came to pick him up, a glove found where OJ climbed the rear wall of the estate with both Goldman's and Brown's blood on it, and a tenant who testified he heard someone climbing the wall at the time the Limo was waiting. OJ got off because of a biased jury, an incompetent, star struck judge, and the best legal defense team money could buy. If OJ had been white and his victims black, you'd be howling at the moon for his conviction and gassing to death.