Cost. That was it. The project in Spain was significantly cheaper to build and was only a 50kw tower. The EnviroMission tower was going to be 200 MW facility. The benefits of the solar updraft tower are huge: Able to be built using readily available materials (concrete, glass, etc.) If a terrorist blows it up, just build another one. Tremendous tourism potential. The skirt/greenhouse could grow crops making the air denser and more efficient, and with a LOT of open land Australia would've been the perfect choice. Unfortunately it just came down to building cost.