when we receive a flu vaccine for a given flu season, we know that this vaccine will not protect us from the flu virus for the following season and maybe even for later in that same season

As you may know, when we receive a flu vaccine for a given flu season, we know that this vaccine will not protect us from the flu virus for the following season and maybe even for later in that same season.  Hypothesize why this is the case. Based on limited information, it is thought that the new vaccines for the current pandemic caused by the Covid 19 virus will be longer lasting. Why would that be the case? how are the two types of viruses different? Id like to see your thoughts without doing any research.