Many of the reasons people already mentioned are correct in regards to why people are so deficient. But there is also an overlooked aspect of B12 deficiency that goes beyond the reasons listed. B12 is synthesized by bacteria, in our guts and and by bacteria that we consume unwittingly every day. With the advent of antibiotics, we have slowly destroyed many beneficial B12 producing microbes inside our systems. B12 producing organisms were also taken in from home grown food. They reside in the soil and are ultimately eaten while ingesting the produce. Because of modern farming methods and the increased use of pesticides and chemicals, the soils are so sterilized that the microbes are wiped out, eliminating a major source of B12 in humans. Our quest to be "clean" has had more then a few unintended consequences.