I can tell you that I'd also prefere if there were subtitles, but there is a reason for there to be voice-over instead. The fact is that most people just listen to the news, instead of watching. People just turn on the TV on the news channel and go on to do something else; also, the most part of the audience of news programs watch it when it's tuned on a TV when they get a coffee, have lunch, etc.
You may have noticed that whenever there is some interesting footage, the anchor gives some warning, they say stuff like: "Here is a video recorded by a bystander/our team was at the place and got some footage of the event/the police have given us a copy of the suspect's mugshot". They say those things so people will look, see what they want to see and go back to what they are doing. Not everyone stares at the TV during the news.
Subtitles would be nice, but they just wouldn't work.