This is the reason given on the wiki:

Policing is a sensitive issue on the internet, and on reddit especially. This causes two problems with our pre-existing rules: firstly, videos of police harassment and abuse are often indistinguishable from political propaganda for one side or the other; and, secondly, the public nature of their office means that the police are often trivially easy to doxx—a term which means 'reveal the personal information of', typically for the purpose of witch-hunting. As you'll see from the above sections, this manages to break all three of our rules so far, and is something with which we have had huge problems in the past, leading to verbal warnings from the admins.

As the outrage sparked by these kinds of videos leads invariably to multiple infractions of our rules against personal information and witch-hunting—as well, often, to the rule against videos of assault—, we do not allow them on the subreddit. There are, as the rule says, subreddits designed for the sole purpose of housing this kind of content, and, as we'll discuss in our breakdown of Rule 9, the size of /r/Videos means that we have to ensure that our content is suitable for as many of our subscribers as possible. Violence of any kind is difficult to reconcile alongside this requirement, and so we try to minimise it where possible for the most part.