Had flu vaccine but still got the flu. why ?

Possibilities: In any flu season there will be at least 4-6 distinct strains of flu circulating the globe. The strains mutate often. At most, they can include 3-4 strains (more would weaken it). It must be given at least 3 weeks before exposure to trigger good immunity if the person is capable. You could easily get a strain not in the vaccine or not have developed adequate immunity in your case.