It's price and value.... young people don't even consider Corvette as an option.
Here are some comparisons. Back in 66 you could get a base model vette with a 327/300 for about 4 grand. A 289 mustang, with some options like disc brakes, etc. would run you just about 3 grand. This put the 'vette in contention with Mustang buyers. You really had a decision to get a loaded mustang or pony up a few more bucks to get the 'vette.
Today, mustang GT is 30 grand. That gets you a 420 hp 5.0. We don't need to go into all the specifics on why the 'vette is better... bottom line is for most drivers both have more power than you need. The vette starts at 50+ grand and comes 2 seats short which is, quite frankly, not practical for most of us young buyers with kids. Growing up, in a pinch, my Dad would stuff more than one of us kids in the 'vette. I would get strung up if I tried the same today. Furthermore, the spread is greatly increased. What used to cost a third more now costs two thirds more. a 66% premium to step up to the 'vette is too much.
Also, the GT is no bargain. I can tell you that my friends that are into performance are not buying them either. Look at Honda Civic SI, Hyundai Genesis, and many others. They offer a fun driving experience, back seats, lots of technology, great gas mileage, and are viewed as very reliable compared to the Corvette, mustang, camaro, etc. And you get all that for 20-something grand.
Chevrolet is competing on the World level for performance, value, etc. with the Corvette. It's great that the car offers great value for the blistering speeds but there are a LOT of us who can't justify that cost for a car that is simply not practical. Back in 66 you could get 4 different engines and that is how they made the car cheap enough for young buyers but competitive on the World level. Now we get one engine that is expensive and requires better brakes and all the other equipment to go with it.
So, until they figure out the price, young buyers are out. There are simply not enough young people making the kind of money it takes to buy a new Corvette. The ones who can afford it may want the back seat for kids... maybe want a more premium car at the same price that is less performance oriented, etc. The list is endless. I can tell you I had a hell of a time in all of my C4's including the one that had under 250 HP. My 66 is bad to the bone and fun as hell. I am young and don't feel the need for eleven million horsepower or being able to outrun a Ferrari. I want a fun car on the weekends I can run through some twisty back roads in. At 50 grand... or heck, way more, the C7 that my Dad has priced out is going to run him 70 grand, this is WAY too much for me to spend on a fun car.