Not only are straight sixes known for their inherent smoothness, but they tend to produce more torque. Also, with a straight 6 you don't need a counter rotating balance shaft to make the engine feel smooth. Also if they wanted to put a turbo on it, there would be more room for it.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member