The way I see it, with the fads of diets and whatnot on what is "healthy" we still manage to be deficient in a whole heck of a lot of things, not just D.
How many people a eat a dinner of a green and/or colored vegetables, with a good fat, with a strong protein, every single day, in and out? No processed foods? How many eat like that every day - lots of vegetables and fruit, high quality protein, few grains, and only complex ones, low or no refined sugar, etc - the type of food the human body is to consume as an omnivore (and if you are vegan/vegetarian, this means always eating your plant based proteins with a grain for a complete protein).
Gut issues affect absorption of many vitamins...how many people have IBS, Chron's, or diseases which affect the ability to absorb fat - and therefore fat solubles? How many people stress their livers with toxic food, when it's supposed to be a filter AND a storage mechanism for fat soluble vitamins (kidneys too).
This isn't even taking into consideration the absolute complete destruction on our environment - our air, water and soil. I used to test most all of the lake, stream and groundwater in the State of Michigan for organic contaminants, namely chemicals classified as "Semi-Volatile Organic Compounds". We have to set the machines at a base level so we don't always report certain chemicals, because they ALWAYS are there. I assisted with Volatile Organics as well for soil and water, SVOCs for soil, and some Air testing. Some things that are always, always in the air or water or soil, depending on it's common state are things like tricholorethylene, various phthalates, toluene, phenols, on and on. I handled - regularly - water that was laden with PCBs, mercury, cyanide, and various other toxic and flammable things....all from water sources in the environment - water sources we DRINK from. Heck, drinking water generally has chloroform in it, but it's "ok".
So my little soapbox aside, why are we jacked up? Because we're poisoning ourselves every single day.
Depressing isn't it....which is why I don't care any more, and no longer work in that field - now I can proclaim ignorance.
/soapbox