A "hard top" convertible would add 1. weight; 2. cost and 3. mechanics may be too large for the Z platform.
"Convertible" tops today are not very similar (in material) to days of old (at least not what I consider "old) and take very little maintenance as compared to what us old farts had to contend with.
While the Z roadster isn't a stripped down vehicle tuned only for performance, it still is a true sports car - why add extra weight just to have a retractable "hard top" convertible? I don't know what the weight penalty is but I suspect it's fairly substantial.
Then the added costs...a hard top convertible would be more expensive to manufacture and more expensive for the consumer; the roadster will already carry a hefty price tag.
You also have to keep in mind that the top has to fit within the current dimensions of the vehicle and while I'm not sure, I suspect a retractable hard top takes up more room than it's soft-top counterpart which may make it a "no go" for the Z.
All things being equal, I'd prefer a retractable hard top as well but at least for me, I don't drive a convertible to have the top up...if the weather doesn't permit top-down driving then I don't drive my convertible.
Nissan already makes a "hard top" convertible for those who prefer that configuration; the G37 Convertible - it's a pretty nice ride...no real need to duplicate that with the Z especially considering the weight/cost penalty.
I'm really trying to decide right now between getting a 370Z roadster or a Nismo but leaning toward the roadster.