Because it's not in line with BMW's current efforts. The engine is a complete revamp of the engine in the 550i. BMW seems to be committed to turbocharging, not SC
If they were to supercharge a V8, it would just add extra cost because it would only be used on one car and be bespoke, like the E60's V10.
BMW doesn't seem to be doing that anymore.
2011 X5 xDrive 35d Red/Black: ZCW, ZPP, ZRC, ZTP, 300, 386, 496, 4AB, 6FL, Combox, LED Coronas. 04/11 Production