From what I understand at the moment the problem is that many police precincts in the US cannot afford them (since they don't get enough funding -- you need to hire people to sort through the footage as well as licensing the hardware and software from Tazer and so on).

Quite a few cops who participated in trials of body cameras were super positive about them (and wished they could keep them) since it also protects them in cases where they are wrongly accused of some sort of malpractice. But in many cases they are too expensive to be able to roll out country-wide and few people want to pay more taxes.