From the manufacturing point of view, the costs of making an O/U and a SbS are the same (all other things being equal). Of course, a classic Anson-Deeley with colour case hardening and lots of hand fitting might be loo expensive to produce now? but the world is full of great SbS designs, that are as easy to turn on a machine as anything. Prior to the WW2, O/Us like Superposed, Rem. 31, Merkel, cost twice or more as much as SbSs of similar quality.
So it's all, as suggested above, about the sales volumes - but the sales volumes are, in turn, a by-product of certain myths shared by gun manufactureres and the general public. Myth #1 is that a SbS has to be expensive to be good. Than't why gun manufacturers believe that an attempt to mass-marlet a SbS at mass-market price is a guaranteed flop. And they are right - because the customers aren't going to believe such a gun will be any good. How many of you reading this are ready to assume that a Spartan 210 may actually be worth its price tag?