Probably because it isn't definite,
They need to look at a number of factors,
Sales - and of what trims
Market response and Feedback
Market Segment for the Z/28 (total track car, slimmed and tightend or HiPo Beast or Beast with cleats?)
Depending on the above then you start your goodies package.
- Integrated RS pack?
- Engine (LS376/480 480hp | LSA s/c 556hp etc - I know the LS9 is an awesome engine, the Camaro wont ever get it)
- Transmission (SS 3.45 axle, CTS-V 3.7. axle..)
and so on....
There is seriously alot of crap GM will have to do to prep and present a Z/28 if and when they decide its economically viable to do so.