Demand, probably, but the cost of the parts -- specifically the shifters -- is a big difference. As an example, my Bianchi Valle was about $800, and the Volpe is about $1000-1100. Almost everything is the same between the two bikes -- I swear that they use the same frame, too -- except the Volpe has Tiagra shifters instead of my Valle's trigger shifters.

At retail prices, the shifters nearly account for the difference in total price by themselves, or even better than paying for the shifters and shop labor to do the conversion. Granted, OEM cost of the components to the manufacturer are a lot less than what we could pay, but I don't think they're willing to reflect that savings in the bikes' retail prices.