Because a V12 would be too heavy.
Although it was revealed at the Detroit Auto Show just last month, an Audi source has strongly hinted that a high-performance version of the new Q7 SUV is on its way. Speaking to Top Gear, this individual stated that "the question is what the better engine would be for a car like this. Would it be a diesel or an FSI (gasoline engine)? Perhaps a diesel, but maybe not a V12 because it's too heavy." However, there's a chance Audi could opt for the 4.0-liter twin-turbo V8 found in the RS6 instead.
That beastly engine produces 560 hp in that car, as well as a 0-62 mph time of 3.9 seconds. We can also expect for the RS Q7 to feature a lowered right height than the standard Q7 along with improved suspension technology and Nurburgring-tuned steering. The bottom line goal here is to make the RS Q7 even more extreme than specific rivals like the BMW X5 M and Mercedes GL63 AMG, both of which are powered by turbo V8s. No launch date was hinted at but an RS Q7 will very likely arrive in just a couple of years' time.