It is because the flu virus is a very large group of very distinct viral groups that are separated by literally hundreds of millions of years of evolution.
Two different flu viruses can be as genetically distinct from each other as you are from a mouse. And to make it even more complicated, they can undergo horizontal gene transfer as well as vertical, which would be kind of like you and a mouse exchanging a couple of genes.
Because they have so much diversity and because they can transfer genes horizontally, each year they can be absolutely radically different.
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