Wow what a statement.....care to expand on that. I am still deciding over the two. I will be using mostly for portraits, family and fast moving kids (which is my issue re the 6D autofocus ability).
Because it is smaller/lighter and image quality is as good if not better. The thing I miss most on 6d is the 8k shutter speed. There is no comparison on focusing abilities, 5d3 is definitely better. In my case the AF is set to ONE SHOT, so I'm not even taking full advantage of what 5d3/6d has to offer. If you will be tracking/shooting moving subjects then you would prefer 5d3.