I wonder if volume might also play a part. Even if they don't use much metal, VTOL minis tend to be much more spread out than a relatively compact tank(especially the rotors). If IWM standardizes the sizes of their molds in some way, you can probably fit a lot less VTOL in a given molding volume than you can tank.
Disclaimer: I know next to nothing about the process, I could be completely wrong here.