The reason, according to Jalopnik, is that the new model has been held up due to EPA certification. Mini told our compatriots, "Sales have dropped because our supply has been so limited. MY 2015 Mini Hardtops have also not been able to go on sale pending the final results of federal fuel economy certification."
Mini goes on to clarify that "as of October 1st, three out of the four 2015 Mini Hardtop 2 door models, the Cooper with manual transmission, the Cooper with automatic transmission, and the Cooper S with manual transmission, have been released for retail sales and deliveries."
So there you have it. Mini sales slumped because, while the new Cooper hardtop launched last year, it wasn't actually ready for sale in the US until a few days ago. In the meantime, supply of the old model was evidently constricted while the Anglo-Saxon automaker geared up for the new model. Now that it's arrived, we can expect sales to return or even exceed their previous levels... or so Mini hopes.