Well, let me say that first of all, I've never competed in bodybuilding so I'm only going on what I know about diet in general. I look at "pre-contest" as an equivalent to eating totally clean for a relatively short time before a show.
So anyway, it's not so much the carbs that are the problem. And it's true that when on a low carb diet, you need to take in a certain amount of fat. However, fat is a concern with dairy because dairy fat, any kind of animal fat, is saturated fat. While it's impossible to avoid animal fat in a high protein diet, the goal is to keep it to a minimum. Thus the preference toward leaner meats such as fish and poultry. This also accounts for supplementation with stuff like flax oil. So, again I'm just making an educated guess here, but I'd have to assume that saturated fat is a no-no before a contest.
Secondly, sodium is always a concern for water retention. Bodybuilders aim to eliminate as much water as possible to get cut before a show. Therefore, unecessary sodium is out as well.