I don't think it's scientists who can't make up their minds, it's the tendency for average people to misinterpret the research results (and in "common wisdom" it's not at all unusual for opinions to vacillate from extreme to extreme - people tend to like easy to remember, extreme, one-size-fits-all advice over middle-of-the-road, use moderation and find-what-works-for-your-body advice).
Scientists did not say "carbs are bad!" they conducted research that found ill effects from excessively high-carb diets.
That doesn't mean "never eat carbs," it means "too many carbs, and some types of carbs (highly refined, super fast digesting ones) can have negative health effects, and very low fat diets aren't as healthy as once thought).
More often than not, research gets taken out of context, because the people reporting the research aren't scientists and they're trying to oversimplify the results, so the media tends to report catchy headlines (like "carbs are bad,") rather than "in our experiment, we found that many people did better with weight loss and some health issues on a low-carb diet."
Sometimes it's like saying "scientists cant make up their minds because some say that vitamin A is bad, and other's say it's good."
When what scientists really reported was the ill effects of both too much and too little.
People hate middle-of-the-road, experiment-and-use-good-judgement advice. People want easy advice that essentially translates into dividing foods into two categories "never eat," and "eat tons of, whenever you want."
I have to cut carbs pretty drastically to lose weight and feel my best (too high a carb diet triggers rashes, pain, fatigue and other health issues), but even having to cut carbs drastically does not mean I see carbs as bad. It's just revised my definition of moderation. I used to see six or more servings of fruit as moderation, now I see it as too much. I used to see three tsp of fat as moderation, now I see six as ok (and 12 is still too many).
It's a lot harder to wrap your mind around moderation, especially if every one may have different needs. People tend to want one-size-fits-all advice so that they don't have to mess with self-experiments or working with a dietitian.