Two points: the rise of ubiquitous smartphones--a phenomenon just twelve years old now--has made it possible to document far more of these crimes. Every police officer knows that somebody could be recording him at any moment; most have the good sense to protect themselves by keeping clean.
If you think in terms of cultural evolution, we would not expect an instantaneous correction of police behavior in response to the new realities. Typically mores require at least a generation to reverse; in this case, we're seeing much faster change. We saw a roughly 20% drop in police shootings in just the last four years.
Juries are part of the cultural system that protects police. These are normal people just like you and I; in adhering to the principle that proof beyond a reasonable doubt is required, they lean towards the testimony of the police officer. It's well-nigh impossible to accept the testimony of an 'obvious criminal' over that of the clean-cut cop.
But do you think that there is any jury in the country that would refuse to convict the policeman who killed Mr. Floyd after seeing the videos?
It is ironic, I suppose, that we have two obsolete forces at work here. On the one side, we have police who still think that they can get away with murder--that notion is obsolete. On the other side, we have people who recall the long history of police getting away with murder and are outraged by a case in which the police officer is NOT going to get away with murder.