Because we can change brain states (ie, manipulating a material object) to go from conscious to unconscious, which is strong evidence for the former. What evidence is there for the latter?
This is evidence that self-conscious beings can stop being self-conscious or that the content of what they are conscious of can change when the stuff that enables self-consciousness to occur is manipulated. It does not prove that consciousness is an emergent phenomena. Suggestions that consciousness is prior to physicalism (and in general prior to the whole division into subjective and objective realities made by a rational self-conscious agent) are not something new and are worth exploring.