First, we must provide a definition of "canonization". For the purposes of this discussion, we will define canonization as formal incorporation into the Standard Works. Only one of the Standard Works is suitable for the incorporation of new revelations; the Doctrine & Covenants. The other three, the Bible, the Book of Mormon, and the Pearl of Great Price, have had their bounds already established. The LDS Church's chosen method of incorporating new canon appears to be by appending Official Declarations to the Doctrine & Covenants, which makes sense since the D & C is already a collection of revelations previously canonized by the Church since its formal establishment. Under this concept, only two revelations have qualified for inclusion as Official Declarations.
Next, we must examine the two Official Declarations to determine why they alone qualified for formal canonization. Official Declaration 1, the manifesto of 1890, indefinitely suspended the practice of plural marriage. It redefined authorized marriage by Church doctrine to mean marriage only between one man and ONE woman. This constituted not just a change in Church policy and practice, but a change in the Church's core doctrine.
Likewise, Official Declaration 2, issued in 1978, also represented a change in the Church's core doctrine. Extending Priesthood eligibility to worthy black men on the same basis as all other worthy men redefined Priesthood eligibility.
In contrast, numerous other pronouncements from the pulpit, such as the change to the single three-hour meeting block, the Proclamation on the Family, the adoption of the Duty to God program for young men and the Personal Progress program for young women did NOT represent a change in the Church's core doctrine; instead, those merely represented changes in Church policies and practices. Thus they were not deemed qualified for formal canonization.
In addition, the LDS Church has made official statements on political questions as well. In 1980, the Church issued a statement against the Equal Rights Amendment which stopped its momentum and triggered its rejection. And in 1981, the Church issued a statement opposing the deployment of the MX missile system in Nevada and Utah out of concern it would transform the area into a bullseye for a Soviet preemptive nuclear attack; the MX scheme was abandoned shortly thereafter. But neither of these statements would qualify for formal canonization because they involved no new doctrine and were merely responses to temporary political questions.
But the point made by Grace for Grace is that although not all changes announced from the pulpit qualify for formal canonization, all are equally revelatory. Grace for Grace writes:
...From what I understand, apostles contemplate an issue, pray for guidance by the Spirit, come up with a decision, then present it to the church for sustaining. On occasion, there will be a letter sent out to congregations from the apostles and read by the local bishop, or the apostles or prophet may read it to the whole body of the church in General Conference (or General Relief Society Conference as President Hinckley did with the Family Proclamation).
This process is also designed to minimize the possibility of another "Adam-God" revelation being sprung upon the Church, which was misinterpreted, resulted in confusion, and ultimately disavowed.
Can you imagine how cumbersome and confusing it would be to incorporate every change in Church policy and practice as an official declaration? It would result in perhaps as many as 50 official declarations appended to the Doctrine & Covenants. We would see 20-30 versions of the D & C in circulation. Changes in doctrine occur much less frequently than changes in policies and practices; it is much more likely that the three-hour meeting block will be changed in the future than it would be for Priesthood membership to be extended to women. This is why changes in policies and practices are redirected towards the Church Handbook of Instructions instead.
Much of what I've published in the post represents my best personal interpretation of what the Church leadership is thinking. I do not know for certain what formal litmus test, if any, that Church leaders use to distinguish between core doctrine vs. policies and practices. But I consider my analysis to be logical.