COST - WEIGHT - DEMAND
The claim is that V6 turbo's are cheaper to manufacture than V8's and they reduce weight by over 100lbs.
Also, demand has been mentioned in things I've read. Most looking beyond 400hp and double that in torque for consistent towing miles over many years, people will go diesel every time. And the occasional towers are not only reluctant but assuming the cost would be darn near equal, diesel wins again.
Also keep in mind that before the Ecoboost and the latest Gen turbo designs with direct injection fuel systems, towing and gas turbos were oil and water...they could not be mixed.
