I think that the fact that we had irrefutable video evidence of it happening (as opposed to e.g. Martin or other "well the officer thought he had a gun" events)

I think that the fact that we had irrefutable video evidence of it happening (as opposed to e.g. Martin or other "well the officer thought he had a gun" events) made it much easier to gain traction in the public consciousness. There was no split-second-bad-decisions that could be claimed (such as when officers shot the man for not obeying conflicting commands), just multiple minutes of sickening brutality.