First of all, it wouldn’t be the Flu (whichever version) if there was no commonality. Second, it’s more than likely we just haven’t found the commonality or understand how to address it. In other words, what exists in all versions of flu which makes flu unique... even single RNA segments.
I understand your point, and am not being critical. I have a background in virology, so it's kind of near and dear to me. Here's a very short summary of why it's hard to make a vaccine that will always work, and against all subtypes: The two major envelope proteins for influenza are hemagglutinin (HA), and neuraminidase (N). There are eighteen different HA subtypes (H1-H18), and eleven N subtypes (N1-N11). Theoretically, any combination of these could be present in a particular flu virus (e.g. H1N1). For the most part, only H1-3 are found in flu viruses that infect people, and usually in combination with either N1 or N2, but even in this case each of these subtype proteins differ markedly from one another, and mutate like crazy. Most of the proteins that meet the criteria for the commonality you are referring to are embedded within the virus - and not accessible to the immune system. There's a reason these guys have survived and thrived for so long.
Have a great night.