I suspect it is because there are fewer and fewer people actually modifying cars. It has become too expensive, a hassle, nobody fixes anything (they R&R) or people just accept the performance the car already has. Each generation cars less about cars and more about other things. Sports, computers, you name it.
When Ford sells 310hp turbo 4 cylinder and 450hp 5.0 V8 Mustangs, most people are fine with the power.
Years ago I said to a friend performance cars are gonna get just like performance motorcycles. Real sports cars are going to get so highly tunes and come with all the power you can use and the regulations to make more power will be so restrictive it won't be worth the expense. Turbos and supercharges will be it, and mufflers. People will say they have a modified car and when you see the car it will have wheels, mufflers, maybe suspension.
The 70's and early 80's were a seismic shift away from hot rodding cars. Cars became, more or less, a way to get from here to there. The 90's and early 2000's slowed that shift, but the shift is moving again now. Baring an unexpected change in interests I predict that within 15-20 years performance cars will be rare. And probably half of them will be electric driven. Look at the interest being heaped on the electric "supercars".
As soon as the battery charging time issue is solved (not if...when) gas cars will be effectively dead.
