An auto. It's essentially a pedal-less manual tranny. T
As for the OP's question, the Viper is "supposed" to be barbaric, that is the idea. They could sell more if they had an auto, or even a V6 option, but the engineers don't care. It wouldn't be a viper.
They could sell 2wd Jeep Wranglers, after all 80% of their customers never shift them out of 4wd anyway, but you would damage your brand/image.
The viper is a "halo" car, it exists because it is what it is, overall sales numbers are less important to it than say a Caravan.