It is unusual for a new model to sell in lower volume than the model it replaces.
17,499 YTD in the US market vs. 20,275 for the same time period in 2016
http://www.goodcarbadcar.net/2011/01...s-figures.html
There are so many factors: production limitations, continued shift to SUVs, increased price and poor lease deals, reduced scheduled maintenance, unexciting styling, lack of an electric model, and mediocre reception in US automotive press are just a few.
Another factor is the increased desirability of lower segment cars. Personally I am torn between an A5 Sportback and a 530i for my next car, with the Audi having reached levels of interior design, refinement and technology that rivals the mid-sizers. If I do not find BMW leases as aggressive as they were for my current and previous BMW, I'm out. The discontinuation of MSDs doesn't help.