Supposedly the DA provided all the evidence. It certainly took enough time and they heard from many witnesses. There was nothing to 'acquit' since there was not even enough to 'indict'. Not even on the least of the charges.
Providing all the evidence is NOT the function of the Grand Jury. That's what I keep saying and folks just do NOT understand.
As Justice Scalia said and is the standard for how Grand Juries function, the DA need only present that evidence necessary for the Grand Jury to reach an indictment. The Grand Jury is NOT the venue for exculpatory evidence and the accused has no right to testify before the Grand Jury.
Further, there was plenty enough to indict. Problem is that there are REASONS why they didn't indict.
1) the behavior of the DA is important. The DA didn't present this high profile case. The Grand Jury isn't sequestered. To delegate such a high profile case might make sense in LA, NY or Chicago, but in Ferguson, MO? Not a chance. He had to be very familiar with the Grand Jury.
2) the DA's office didn't present the case. The DA's office presented the Grand Jury with a bunch of evidence, much of it conflicting, did NOT confront Officer Wilson on his various testimonies and did NOT proceed through the various counts and WHY the Grand Jury SHOULD indict.
3) The DA's office specifically presented a LOT of exculpatory evidence to the Grand Jury without explanation. This, coupled with NOT stating WHY the Grand Jury SHOULD indict made it quite easy for the Grand Jury to return No True bills on all counts.
So I don't repeat myself ad nauseum, it'd probably be better to just read my post above on what the Grand Jury finding is and isn't.
Your misunderstanding of this is rather common. Unfortunately. But to be perfectly clear, there WAS plenty enough to indict if the DA had wanted to indict. Even a little bit.