HepA, HepB, norovirus, polio and small pox never mutate?
Its a good thing we were able to wipe our small pox before it mutated and made the vaccine ineffective.
Why were we able to eliminate small pox and have corned polio, but flu requires a yearly vaccine.
See, you're very smart because you have asked a very important question. But why is your first reaction "this is all a conspiracy!", instead of "surely there is a reason this happens let me go find out why".
Short answer:
Every virus is different, and viruses like influenza and rhinovirus (common cold) use RNA instead of DNA as their genetic material. RNA is much more susceptible to mutation (copying defects) than DNA, as such viruses based on RNA mutate much faster, and have much more varied mutations than viruses based on DNA.
Influenza is also unique in that it can also easily borrow genetic information from other influenza strains. If two different strains affect you, they can mix and match their genes and create a whole different virus. These changes are usually so drastic our bodies have a much more difficult time recognizing them and fighting them.
Again, influenza is very unique in that it can affect multiple species. Usually a virus (like polio or chicken pox etc) affect one species like humans. Influenza though can infect birds, pigs, monkeys, humans, etc etc. With so many possible hosts the virus can spread and mutate much quicker.
With viruses that mutate slowly and stay in one species, you can inoculate enough of the population so that the virus has no hosts to infect. With no hosts to infect, these viruses actually die out completely. With a fast mutating virus like influenza, by the time you inoculate everybody to every strain in existence, it has already mutated into something else slightly different and it can still infect hosts and continue to survive. Thus it becomes near impossible to eradicate, but with current science levels, we can predict and react to limit its effectiveness.
As you can see Influenza is a very unique virus, unlike many other viruses we come in contact with. Yes this is the short answer, if you really want to know why influenza is different, rather than remain ignorant, go google and read up on it.