That would only makes sense if vitamin D is the only one that has any real significant effects or if the other ones who do, are too included in small dosages (this doesn't seem improbable at all).

I remember seeing studies which doubt that vitamin C would help healing from common cold. No wonder if most other are as insignificant.

Also, just checked some pills of vitamins (for hair, skin and nails) I bought 1-2 years ago. It says "take 3 times a day" and it has 100 IU of vitamin D. It's also apparently 50% of RDA - most other vitamins/minerals in it are up to 200-250%, and my vitamin D pills are 1250% RDA. Mystery solved, I guess.