Without wishing to cause offence ;-) I suspect that the colour may be part of the problem too, in addition to the other things people have mentioned. Time of year will also play a big part as private buyers will be few and far between at this time of year, I am sure.
I'm not biased against the colour, I have a Crimson Red E90 and it was the colour I was determined to have (although white was tempting). I knew it was a less-than-mainstream buy when I chose it and that was reflected in the price for my pre-reg car. The majority of people I know, admittedly non-petrol heads, would not have chosen a red one although it gets compliments on the one day per fortnight that it is clean.
I'm still hoping that I am ahead of the curve, given the number of F30 press demonstrators in red, but suspect I will have moved the car on long before red really is "back in fashion" for large(ish) cars.