He was like an interim Hokage, only acting in the role while Tsunade was unconscious. Never fully instated.
Kakashi is the real 6th Hokage
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Yeah maybe if tsunade had died while he was in office he would have been officially the 6th. Notice though how kakashi became the 6th immediately after danzo died. I don't think the story really has a logical reason why danzo didn't become hokage other than the writer didn't want him to.
No matter how gaudy you make your appearance...it is only a facade..
*Obama/Biden/Harris Elected* Hopes Rise..Fears Shatter