I also live in the south and the reason we slab first down here is because it's swampy. No basements. I think in that climate up there it's not necessary to start that way, whereas down here if we don't start on a slab the foundation will never be stable. Even with slabs we have serious foundation issues. Having done some construction work before, I would rather do flooring work indoors than outdoors if that makes any sense. Perhaps that's why it is done towards the end. Easier that way? Just guessing though.

Building 2 will absolutely have a floor though.

FL