I'm gonna go on a limb and guess that the reasons they'd consider the wing over the tank are these...
-The flying wing can be scaled down easier to fit the $20 assortment because it's essentially one shape you just need to make smaller. Not much to it then.
-The tank's a more complex vehicle in design than the plane... More possible moving parts and things, more overall "detail" you'd need to add to it perhaps.
I'm just guessing on those things, but they seem feasible to me.
As far as the tank goes though, it's not really accurate to call it a "Nazi" tank since it was actually given to the Germans in TLC by the host country they were in (in exchange for a RR Phantom?). Also, the tank is actually modeled after a "German Mk.7" tank from World War I, though Mk.7's were British tanks repainted by the Germans.
Either way, the tank's actually from a whole other era completely...
Personally I think the tank makes more sense in the $20 scale because it is indeed smaller and it's more action-oriented what with the fist-fight on it, but Hasbro's mentioned somewhere in the past how they need to feasibly scale-down a ship or whatever so it looks believable, but fits the pricepoint... Maybe that's playing into this?
The AT-AP is much more comical looking compared to a fighter for instance... The tiny hatches and things really make it look silly to me by comparison. Maybe that's a reasoning for this?
Either way, I agree with speedermike completely... Why not just release a $30 assortment with some of the larger vehicles?
As always the scale would be important to me, but hey at least it'd get the things out there to people who want them.
I think the flying wing will look odd myself since Indy and the German thug fight under it, but whatever.