Why is a new flu vaccine made every year?

Because there are new mutations of the disease and immunizations must keep up with new strains that are resistant to old vaccines. There may also be some kind of biological aspect in the human body that limits efficacy to one year, but I'm not sure.

A new flu vaccine is made every year because the strain changes and mutates to have more dangerous. Personally, around flu season there is 1 primary strain that breaks into multiple. That is why the flu shot doesn't 100% protect you against the flu, because it can mutate and your body can't fight against it because it doesn't know how. The flu is a virus, which means there will never be a 100% cure, only medicines to treat the symptoms.