My B12 was 98 My doc gave me 10 methylcobalamin inj. And told me take one every month for 6 months and one every year. Can u tell why?

Poor absorption. It is likely that your intestine was not absorbing B12 normally and it is an essential vitamin and it is important to have a blood level above 250. Please do follow your doctor's advice and keep taking the injections.
B12 deficiency. your body can not absorb the vitamin B12 from your stomach .The only way your body will get its need for vit B12 will be by injections the body needs about 1 microgram of B12 /a day. the injection is 1000 microgram. .the first 10 injections needed to fill up the tank (here it will be the liver) after that the once/a month for replacing what you consume.
B12 deficiency. You clearly has B12 deficiency. Do you have neurological symptoms? Was you evaluated for the causes of B12 deficiency. Oral B12 and injection are similar if you absorb well. But, we prefer treating severe B12 def and symptomatic patients with B12 inj first. Once patient's symptoms resolve and B12 store recover, maintenance therapy can be continued with either monthly B12 inj or oral B12.