The stomach acids that aid in the natural breakdown of food also breakdown vitamin B12.
The body will only absorb a small amount of the nutrient from a pill.
In addition, as you get older your body's ability to absorb nutrients through digestion continually decreases. In fact, many adults are unable to absorb Vitamin B12 into their bodies at all!
Injections provide a direct method of supplying your body with the necessary Vitamin B12.