Because a V8 can make the power without the turbo. And weight difference is negligible considering the weight of modern cars.
Also, 6cyl engines sound like crap. Even the very best one's only sound "okay.">>17710952>why don't any turbo 6's race in v8 supercars?
That's changing starting next season. Also, they were used until the early-'90s until they were banned.