I would like to think that their reasoning is much the same as that which dictates that we won't get Jean-Paul Valley as Batman; namely that to the general public Peter Parker IS Spider-man or Bruce Wayne IS Batman and that (as far as the studios are concerned anyway) most people have no real interest in another character assuming that mantle. I would like to think that it's not down to race.
Having said that, audiences took to a Zorro who wasn't Don Diego De LaVega and TDKR ended with another character assuming Batman's mantle (though he hasn't gotten a spin-off movie and presumably won't, given that Ben Affleck is now Batman/Wayne).
It might also be interesting to see what happens if Sebastian Stan's Bucky Barnes takes over as Captain America or Don Cheadle as Iron Man. Will this prove that audiences are interested in the superhero mantle, not the man behind the mask? And if so, is the ball back in Sony's court?