To understand why vitamin D supplements find such widespread endorsement, you need to take cognizance of the fact that vitamin D is one of the nutrients we really need in our bodies. It is the nutrient which, among other important functions, regulates the levels of phosphorous and calcium in the body. The upshot is that without adequate levels of intake, one is bound to have calcium and phosphorous imbalances, with dire health consequences.
Having taken cognizance of that fact, you further need to take cognizance of the fact that vitamin D is, arguably, the vitamin with the least number of potential sources. True, you can get all vitamin D you need from the sun. But if you are unable to get exposure to the sun (and you need a good amount of that, to properly tap vitamin D), then you would have a very limited range of alternative ways you can get the vitamin. For in terms of diet, the foods that can give you vitamin D are foods that you can count on your hand: fatty fishes, cod liver oil, beef liver, eggs and mushrooms. Those are literally the only foods from which one can get vitamin D.
What emerges from all that is a clear testimony for the fact that it is highly unlikely for the person living in the temperate parts of the world to get an adequate supply of vitamin D from either the sun, or from dietary sources.
It is from such a background, then, that vitamin D supplements find so much endorsement, because everyone sees that they serve a useful function (of providing the people who make use of them with a nutrient they would probably have ended being deficient in). That, by the way, is hardly the case with most other nutritional supplements. The reason as to why most other nutritional supplements have difficulties getting endorsement from the mainstream medical establishment is in the fact that such other supplements are normally meant to supply people with nutrients they already have an adequate supply of; leading to serious question being raised with respect to what the supplements are actually being taken for.