Nibbana means extinction, doesn't it?
Kilesa-parinibbana being the extiction of defilement, khanda-parinibbana being the extinction of the aggregates.
Such a state (if it can be called that) is asankhata because there is no falling back from it. The extinction (of kilesa/khandha) is final.
Space, and other elements are conditioned because it is possible to attain them, as meditative states, and yet fall back from them. They are supported by conditions and so change accordingly. Nibbana is not a state of becoming, but the end of them. Not something built up, but an ending of the process of becoming, fabrication ect. based on ignorance.
"If beings knew, as I know, the results of giving & sharing, they would not eat without having given, nor would the stain of miserliness overcome their minds. Even if it were their last bite, their last mouthful, they would not eat without having shared."