that still requires a fancy , multipart mold and additional equipment .
More fancy than just making a multipart mould for plastic, I would think, because they now need a metal moulding machine as well. (Unless they outsourced it, of course, but I kind of doubt
Tamiya would.)
But the odd thing with lower hulls in general is that Tamiya and Italeri were making them in a single piece in the 1970s and '80s already, but it wasn't until the late '90s or so that Dragon began making a lot of noise about their "slide moulding" technology that was nothing more than what Tamiya and Italeri had been using for those hulls for decades. And in recent years, the hulls in a lot of AFV kits are no longer one-piece but need to be built up from flat plates — even from Tamiya.
Why do they do it in a zinc alloy ?
I've been wondering about that since I read about the first releases in this line when it was new. I guess it's to give the model some weight, but I can't see the point of that either, because who cares if a model has anything approaching scale weight?
Dragon also uses a metal hull in some of their 1:72 scale armour kits, but in that case it's because those are/were also sold ready-assembled, as well as in kit form. Using a hull with as much detail as possible moulded on then saves expensive labour costs at the factory.
was Tamiya the manufacturer that included a slab of metal in some of those smaller kits for added mass ?
If I'm not mistaken, in later 1:48 scale AFV kits, they've gone off the metal lower hull and use a plastic one, but also include a simple metal bar to put into it to give much the same weight.