It just seems to me as if people default to it, like they would default to Stairway to Heaven or something. Each of the Beatles have a song with radio play (I hear Octopus Garden every once in a while) so you'd think they'd know more than two of them. It also depends on the era, because my older relatives like IWTHYH and Help, but I also really like Help so I can back them up on that..
I also have problems with the people say "Live and Let Die is my favorite Beatles song!" or some uneducated thing.