THIS GUY NAILED IT.
I'll throw out my professional opinion as an Industrial Designer.
It's because it's ugly and outdated. There have been, now, 3 major revisions of the Corvette since the Viper debuted in '92. While each has kept certain design cues that make them 'vettes, they've been major redesigns over the c4 (which ran until '96, 4 years after the viper launch).
In '92 the Viper was extreme by comparison to the fairly tame vette and higher priced Ferraris and Porsches. It had no exterior door handles, roof or windows. no driver aids, and a freakin V-10. Even when the released the coupe in '96, it was still looked great compared to the C4 and even when the C5 hit, the viper still seemed so much more over the top that it kept it's image, all while it's ******** nature was being dilluted with subtle changes like windows, rerouting the exhaust, etc.
When the first complete redesign came in '02 it looked great, but the beastly nature of the first gen had been replaced by an almost over-refined, yet still monstrously powerful sports car. If I were to choose a Viper based on looks alone, i'd pick this generation. While it maintained certain cues of the first gen, it progressed the look very well. But over the next decade, with the progression of the Vette, 911 and almost every european sports car, even this Viper began showing it's age, and it just wasn't extreme anymore. Other v10s were hitting the market from Audi and Lambo, the definition of extreme had been pushed so far by Bugatti that the Viper (while much less expensive) seemed mundane.
So when the current gen came out, I had hoped for the complete rebirth of the car that was so needed, rather than simply the same-old, same-old look we got, this time bringing back the body lines of the first gen, but a ridiculously cartoonish hood and nose treatment. I'd have accepted a well refined gentlemen's sports car, or all-out insanity on wheels. Stylistically, we got neither. No imagination, no poster car for a 12-year-old's wall, no beast.
The first Viper was purpose built around a monster of an engine. The current, seems to merely exist until they think of something better to do with it. Yeah, it's got power, but like everyone's saying, it's not Hellcat power. So where's the insanity? In a time when a lesser family sedan can have as much HP as the first gen Viper, it's hard to think of the current viper the same way we thought of the first.
A next gen viper needs to be so over-the-top, toddlers cry at the very sight of it. If the Hellcat Charger/Challenger is going to top 700hp, the Viper needs to hit 900 minimum. 22 years of history aren't enough to lock in it's core design language the way the Vette has, for the viper to survive, it doesn't need to evolve, it needs a complete rebirth. If the next next zr-1's going to be mid-engine, do it first. The fact that the hellcats exist, tells me that the time is finally right for the Viper to get the redesign it deserves, and we can only hope they get the picture.