The philosophy behind an M car is to have a high revving engine and a balanced chassis capable of sporting drives. These characteristics aren't fitting for a 7, according to BMW M GmbH VP Gerhard Richter. It would be sort of...not right (for lack of a better word) to have a "sporty" 7 series.
Introduce Alpina, a longtime partner with BMW and a highly trusted tuner. Alpina uses forced induction (specifically superchargers) to achieve equal outputs to their high revving M counterparts. An Alpina usually is more of a relaxing model, more fitting to the character of a 7. The B7 is essentially what an "M7" would be. The suspension is modified to handle the 500 or so hp that the supercharged 4.4L V8 produces, and its much easier to use the meat of the power in day to day situations. Think of it as a compromise between what an M7 would be, and a 760Li.
Past:'08 E92 335i|ZPP|ZSP|6AT|four (4) HPFPs
Present:'15 Mustang GT|401A|PP|6MT|Recaros
Sig pic unrelated. I like to go sideways and stuff.