Influenza is localised to the lungs, the virus attaches and multiplies there. From there to lymph nodes in any discernable concentration is a long way. Tetanus is a main circulatory system infection so it will come into contact with lymph nodes more often and in higher concentrations allowing a greater immune response.

You need more top ups of influenza vaccines to increase the possibility of an immune response from the lower concentrations of virus entering the lymph nodes.