Keep in mind that most people in the U.S. care about issues like this in an abstract way. They think that improvements should be made, but when, say, the time comes to vote they won't bother taking 10 minutes to see where their representatives stand on this issue and vote accordingly. If you ask people who they voted for the day after a primary, most will have a hard time even recalling the names in the downballot races, let alone what the positions were. The people who do tend to vote for these positions tend to be extremely focused on particular policies - they only care if the politician supports or opposes some obscure regulation that benefits them personally.
So in the end you really are talking about a very small number of activists and groups that are trying to bring about change, and they're fighting against entrenched groups opposed to said change - or more often than not, just general political inertia (why rock the boat if most voters don't care?).
There has been traction in the effort to get more body cameras, though, so there have been some improvements. But there doesn't yet seem to be enough people who care and act accordingly to bring about the big changes that are needed.