There is a simple reason. While the SS was historicaly the top of the line street HP car because it was always marketed that way. You could order them as hot as your wallet could handle. The first gen Z/28 was a purpose built option that had only one reason to exist. They had to make it available to the public in order to claim it was a "stock street vehicle" when in reality it was really just a homologated race car.
The Z/28 was part of the "Win on Sunday - Sell on Monday" concept. If you remember the great sucess that Ford had selling the original early-mid 60's Mustang you will understand why we even have a Camaro at all. The Camaro is a direct result of the Ford having the Mustang.
The Mustang was actively competing in SCCA sanctioned road racing events in the 5 liter and under stock class. Chevrolet wanted to sell a lot of Camaros and they needed to win at the Mustang track and at the Chevy showroom. The way to compete against them on a level playing field and show all the potential customers who was the best car to buy was to enter that same 5 Liter class then beat the snot out of the Mustangs. They did exactly that.
The Z/28 is the Champion with the winning sactioned racing heritage against Mustangs to show for it.
While the SS has always traditionaly been a top dog street bruiser, it does not have the same race winning heritage as the Z/28.
-Mark.