Suppose you put hardware cloth or even chicken wire over the top to keep it light weight, and make arrangements to cover all or part of it with canvas, or a tarp, for protection from sun or rain, and keep it lightweight? I mean, isn't the whole purpose of a tractor its portability? Once you start using plywood, OSB, shingles, metal roofing etc., you are no longer building a tractor but a permanent structure.
On the same vein, if you cover the bottom with hardware cloth and dirt you prevent the chickens from accessing the grass beneath, which, again, is the function and purpose of a tractor. You want to be able to move it around so the chickens can graze in different parts of your property. Put a floor in, and you've defeated the purpose. Plus, again, added considerably to the weight.
Seems to me, anyway. Not meaning to be critical here, just my observations.