Personally, the characters are what make the Pretenders prominent and I find the very concept of pretenders flawed and in need of reworking. If the gimmick of your toyline is that you can completely ignore the existence of half the package, it's not a very good one is it? Maybe if putting the robots inside the shells activated features of some kind, there'd be a selling point but as it is, if you can't even tell if the robot is inside, what's the point? Schrodinger's bot is a decent thought experiment, not so much a product.
Also, now that I think on it, Hasbro may not be willing to dilute the brand by populating the toyline with knockoff Centurions, bootleg Ninja Turtle mutants and He-Man rejects for a few years.