Inline 6 in most cars would throw the weight balance off, making the car pretty slow. Look at Jeff Jones' car from last year, an S13 hatch with a 1JZ. That car was slow as hell. The advantage of running an aluminum V8 is it's weight, and how much of that weight sits behind the front wheels. That plus the torque and cost effectiveness is why everyone goes V8.
I understand why everyone loves the 4 and 6 cylinder engines. It's cool to see David slay Goliath, but in a competition you want every advantage you can get. Plus, everyone running V8s has made Formula D the amazing series it is today. It's super competitive. Wouldn't it suck if one or two guys ran V8s and everyone else was in 4 and 6 cylinders engines, and the only guys running V8s won all the time because their cars were so over engineered?