Why aren't all vaccines required annually?

Immunity persists: Many vaccines lead to immunity for much longer than a year, so it is not necessary to repeat the vaccine. The influenza vaccine is given every year because there are generally different strains of the virus that cause the flu each season. So, it is important to get immunized against the new strains.

Good question: Most illnesses that vaccines protect against are caused by viruses with fairly stable target antigens (target antigens are what vaccines stimulate your immune system to recognize and destroy) that do not change from year to year. The immune system typically has a memory that lasts for at least several years after a vaccine is given. Influenza virus is unique, target antigens change more rapidly.

Biological memory: For some viral germs like measles that never change, the body holds memory of which antibodies to make to prevent it for decades if not a lifetime. Shots for some bacteria like tetanus, seem to do this for up to 10 years.Some germs like the flu adapt/change their form every year or 2 and yearly vaccines are more helpful. That is the wondrous way a healthy body works.