The TBM answer: We can't get into Brigham Young's mind. We don't know what he meant (Actual answer from my TBM Institute- director brother).
The FLDS answer: Because the Adam-God Doctrine is true. It was after the Church abandoned the Patriarchal order of Marriage that they fell away from many glorious doctrines such as Adam-God
The Exmo answer: Because it's all just made-up stuff.