Dare I suggest that the reason that neither G.I. Joe or Barbie got the top spot is because, for the most part, they're both gender-specific toys? G.I. Joe, of course, being a "boy's toy" (some present company notwithstanding), and Barbie being a toy for girls? So, they make them numbers two and three, and give the number one spot to a more universal, unisexual toy like the hula-hoop, which everybody can enjoy (although I enjoy hula hoops much more when they're being played with by members of the female persuasion).
And, the last time I went to my neighborhood park, I found a hula-hoop that someone left behind. So, somebody's still playing with them. Either that, or that park has a terrible maintenance staff.