[Crazy Jetty]

I think it's a bit of both, honestly. It logically and undoubtedly is the star characters (The four turtles, Optimus & 'bee, Batman and Superman, Ironman) who sell best to kids. Anyone who doubts that really has no clue how a child's mind works. But it also is a self fullfilled prophecy that when the ratios are 90 to 10, it only compounds that.

When Hasbro/Kenner was doing the Batman: TAS line, there would be four, five waves in a row that was literally nothing but batman variants before we'd get a robin, or a villain. So of course Batman will outsell everything when there literally is no other option.

But this mindset also comes to a detrement to the kids, as well. Sure, the one off kid who just wants a cool toy will only want spiderman and not give two shakes about anything else. But the kid who truly loves Spiderman will not only want Spiderman, but he's also going to want Peter parker (Preferably just parker, with no odd costume change gimmick, or Spidey unmasked), J. Jonah Jameson, Mary Jane Watson, Aunt May, Eddie Brock, Flash Tompson, Norman and Harry Osborn... and every single villain in his rogues gallery. Because he can never really play out his own spiderman adventures without all those important supporting characters.
It's even worse for Superman, as Supes doesn't really have many memorable rogues, and 85% of who and what he is revolves around Clark Kent, and the men and women in his life.

It's always a given that any toyline outside of Star Wars is going to scew to the heavy hitters (Where as Star Wars, even aliens that were on screen for three seconds and never seen again in any other source of fiction are still guarenteed to get a toy if you wait long enough), even kids accept that fact (I know I did). But superhero lines seem even worse about it.

I don't even know what I'm trying to say anymore.
I think it's both. The big names sell better naturally, but they're also digging themselves into a pit when they offer no alternatives.