This is a great question, but also one no more complicated than why would you buy an apple when you could get an orange?
They are two different means to different ends. The Z/28 is not the fastest track car GM makes. Plane and simple.
Why would you buy a ZL1 when for the same dough you could have bough a Gran Sport Corvette?
Because some people love Camaros and some like Corvettes. A few people will go back and forth for what suits them at any given time. But some would prefer the fastest Camaro more than the 2nd fastest Corvette.
I wouldn't throw either out of be for eating crackers and if I were economically of the appropriate means, I would buy every single car that has an LS7 in it. Of course if I had that kind of cash an SS sedan with an LS7 would fill out the fleet.
When we see the numers, I am pretty sure that the Stingray with 450 PLUS HP and the appropriate cooling package will run all day and then some with a Z28. That's simply because it will weigh a good 500 less with an engine that will be only a bit short of the LS7.
So the question is not just the Z06, which costs more, but for the mone a Z/28 is likely to cost, what else would float your boat?