Because the police have an extraordinarily strong lobby and our elected representatives aren't interested in protecting our individual liberties.
A video recording is different because it completely shatters the long-held power that the police had of being believed. I'll never forget learning that simple truth in college when a police officer wrongly accused a friend of mine and I of doing something completely fabricated. We had our description of the actual events, the cop had some made-up story. Guess who the judge believed?
Cops have had the protection of belief for way too long. They've abused that power and even with pervasive video, they will continue to abuse that power.
Fortunately, for cases when it is available, video is the great equalizer. Cops don't like to have equal footing with citizens. They want to be right. They want to be believed. They want to have all the power.