While a strict vegetarian diet can cause B12 deficiency, it takes very little meat to provide adequate levels of B12. The cause of B12 deficiency is not usually dietary (except possilby in strict vegetarians).

B12 deficiency can be caused by many things. The main cause is thought to be the autoimmune disease pernicious anemia, where one lacks the intrinsic factor to process B12. You could eat an entire cow (little exaggeration here ) and not get the B12 your body needed, because it cannot process it, if this is the case.

Other causes can be gluten sensitivity/ celiac disease (also autoimmune, the intestines are damaged and cannot absorb B12), over use of acid blockers which also block absorption of important vitamins, low stomach acid, h. pylori infection, inborn metabolic problem, tapeworm, some medications are thought to deplete B12, like metformin, birth control pills deplete B vitamins, nitrous oxide can deplete B12, and more.

Forgot to mention, the liver stores enough B12 to last for 5 years, so by the time someone shows a vitamin B12 deficiency... it usually means there is something of a long lasting nature going on to have gotten there. It is another reason why diet is rarely the cause except for very strict vegetarian, because one can typically rely on their 'stores' during periods in between eating dietary sources.