As someone previously mentioned, the Schneider XXL series is based upon the basic Dagor design with modern coatings entered into the lens equation.
Talk to a lens designer about a Dagor and you hear that you can easily do technically much better for the dollars expended in producing it. As Jim mentioned the market for 19" to 24" Dagors (or their modern equals) is very limited and one would have to produce many (hundreds) to keep the price within the range of the photographers that would want to use them. Look at the suggested retail price for the Schneider XXL series (the 550 mm and the 1100 mm) and you will get a sense of the numbers I am making reference to.
That is what makes them scarce commodities in the used market.