Because they are absolutely tried and tested, in fact there is a whole forum dedicated to tuning them http://www.d-series.org/forums/engine-building/59924-d16z-turbo.html With people achieving well over a 1000 bhp. Obviously the K20 is a superior motor, but don't discredit the D16 motor as they are seriously capable motors! They're cheaper to get a hold of, they are very common and you're not paying a premium for having a K20. Tuning for these are very well understood. The non-type S type R's already have a D16 motor in them, it's just a matter of swapping it for the ideal variant. It's the choice I would make for this car, but each to their own.