Clutter found this so far but there is more and I will keep looking (didn't bookmark it unfortunately). It is a bit long so skip over first bit as that is just stating the obvious:
Why do I need to get vaccinated against the flu every year?
Flu viruses change from year to year, which means two things. First, you can get the flu more than once during your lifetime. The immunity (natural protection that develops against a disease after a person has had that disease) that is built up from having the flu caused by one virus strain doesn't always provide protection when a new strain is circulating. Second, a vaccine made against flu viruses circulating last year may not protect against the newer viruses. That is why the influenza vaccine is updated to include current viruses every year.
Another reason to get flu vaccine every year is that after you get vaccinated your immunity declines over time and may be too low to provide protection after a year.