I think there is 10-15% of folks telling a bit of a fib here.
What is it, does MT have more "street cred". For the E92 M3 the split between MT and M-DCT in North America was almost precisely 50-50 (
link). I strongly suspect it will be close to the same with the M4.
For those that really care about going the fastest possible (in a given car, of course) don't forget that a M-DCT is equivalent to at least 20 hp, perhaps as much as 40 compared to a MT. If the driver in the MT can not shift supremely fast (most of us) the number is closer to 40 hp.