When you are dealing with engines as large as the Vipers, moving to a DOHC would add a LOT of weight to the car, and move quite a bit of weight higher on the engine, going against the desired low-as-possible center of gravity. It is not possible to lower the Viper’s engine any more in the frame rails.
Packaging is also a MAJOR concern. The Viper’s 8.0-8.4L V-10 is not much larger than many engines half its displacement, and is light as hell for its size, allowing you to fit a gigantic engine under the hood without having to make the car bigger to accomodate it. The Viper engine, also being a 90 degree V-10, is quite wide. With the addition of DOHC, it would not fit in the current body, requiring the car be made at least 8 inches wider, adding yet again more weight. Another option would be changing to a 72 degree DOHC V-10, but the engine height then becomes an issue. All of these possibilities have actually been investegated by SRT.
As everyone else stated, simplicity. There is a reason that many quad cam super-cars are a pain in the ass, they have a ton of moving parts. Pushrod engines are bulletproof, and power the fastest cars on the planet.
HP/Liter is for wankers.
HORSEPOWER matters, who gives a shit how you get it if its usable.