Not sure, except that it is generally true of the entire DC series, at least in my observations over the years. Among the entire DC line, I've found the DC10 pops up the least often; and the DC3 and 5 coming in about even after that, which is still not all that often; and I have never once seen a DC2 on the market block. My suspicion is that not nearly as many DCs were made, and they certainly did not enjoy the marketing success any of the Marks did. FWIW, once you find one, I think you'll be very happy!
Edward