Because the law doesn't require a prosecutor to present exculpatory evidence to a Grand Jury. The Defense isn't even involved, just a prosecutor selling his case with no rebuttal. This is why they say "you could indict a ham sandwich". The evidence was not withheld from the defense during the trial.
I can't believe, even in Austin, that there wasn't one person on the jury that refused to convict this man.
Thank you for the explanation as to why. Yeah, it seems odd that all voted to convict him.