My argument has always been:
If an Audi supercar can sell, why can't a BMW supercar? And BMW certainly has the technical know-how to do it...
Though I think BMW believes its more profitable to sell the "common" cars. They are rapidly expanding their range for buyers of all demographics... Though hopefully something is in the works, i.e. the "Green" supercar.
Well, if you really want a BMW supercar. Get rich and get on BMW AG's board of directors