Probably because mass consumption of carrots isn't a top public health issue? If consumption of carrots rivaled that of soda I'd be willing to bet there would be more studies on it just the same as there are studies on various other foods such as red meat, egg cholesterol, fish oil, etc.
Actually mass consumption of carrots can cause your skin to develop an orange tint (seriously, not joking here).
Heh. My brother's wife was starting to look like an Oompa Loompa because of her carrot snacking habit. It was pretty funny, though it doesn't seem to have had any deleterious effect on her health.
Same with yams. It happened to my brother when we were kids.
How much of which jam did he eat and what did he look like? (colour/intensity)
In most parts of the US a sweet potato with soft orange insides is correctly called a "Yam" to differentiate it from the sweet potatoes with firm yellow insides.
Either name isn't great; it's neither closely related to the african yam, nor the common potato.
...and is being used by people who try to tan their skin.
Are you joking? I can't tell.
I sure hope so...
It's because of old-school research methods. They haven't yet developed a method which correlates any diet with any effect, so they have to look for correlations from a specific subset of data, which is assumed to return results.