Because for Mormons, it is not - and has never been - about doctrine per se. The entire LDS system is about claims to authority and succession. Doctrine (or more properly, the altering of it) is part-and-parcel of the long-term LDS experience. "New revelations" and changes to "official doctrines" are simply mechanisms used to test the membership, to see if they're continuously paying attention to/obeying the current authorities, not just the past ones. It's not about truth or doctrine in their system - it's about making sure your membership is current.
I disagree in part. It's about trying to ascertain that every soul, whether living or dead, is confirmed a member of the mormon church and subject to the laws and ordinances thereof, in the process filling the earthly coffers and bringing the souls of those who died under the command of Joseph Smith.
". . . no man or woman in this dispensation will ever enter into the celestial kingdom of God without the consent of Joseph Smith.
Mormons take the law of consecration seriously enough to take this oath in their temple ceremony, "You and each of you covenant and promise before God, angels, and these witnesses at this altar, that you do accept the law of consecration as contained in this, the book of Doctrine and Covenants, in that you do consecrate yourselves, your time, talents, and everything with which the Lord has blessed you, or with which he may bless you, to the Church of Jesus Christ of Latter-day Saints, for the building up of the kingdom of God on the earth and for the establishment of Zion."