In a private letter to Eugene England written February 19, 1981, McConkie stated (emphasis added):

Yes, President Young did teach that Adam was the father of our spirits, and all the related things that the cultists ascribe to him. This, however, is not true. He expressed views that are out of harmony with the gospel. But, be it known, Brigham Young also taught accurately and correctly, the status and position of Adam in the eternal scheme of things. What I am saying is that Brigham Young, contradicted Brigham Young, and the issue becomes one of which Brigham Young we will believe. The answer is we will believe the expressions that accord with the teachings in the Standard Works.

Later in that very same year, McConkie would publicly state:

there is not one of these that I have ever known to be taught in the Church … I am giving you the list … Adam-God theory …

Why don’t more members know about Adam-God teachings?

As documented by Corbin Volluz, when Elder Widstoe compiled Brigham Young’s teachings into The Discourses of Brigham Young in 1925 he deleted certain Adam-God phrases. The 1997 priesthood/relief society manual and Teachings of the Presidents of the Church pulled from Widstoe’s book and not the original discourses.

And Boyd Kirkland, author of multiple essays/articles on the Adam-God theory, related this incident in a letter to the editor of Dialogue (Dialogue 31:3, 1998):

I wrote a letter to President Spencer W. Kimball in the summer of 1980, asking why he, as well as Mark E. Petersen, Bruce R. McConkie, and other general authorities, had been so vocally denouncing the Adam-God doctrine, while at the same time denying that Brigham Young had been the source of the idea, when there was an abundance of good evidence to the contrary. [For example, President Kimball said in the 1976 general conference, “We warn you against the dissemination of doctrine which are not according to the scriptures and which are alleged to have been taught by some of the General Authorities of past generations. Such, for instance, is the Adam-God theory.”]

I pointed out that this approach created a double dilemma for church members aware of the facts: first, how a prophet (Brigham) could claim as revelation and promote to the church an idea deemed by later leaders to be a dangerous heresy: and, second, why later church leaders would dishonestly deny the true source of the “heresy,” claiming it originated with “enemies of the church.”

I indicated in [a letter to the First Presidency] letter, that I felt this dilemma was simply the result of a misunderstanding or lack of information on the part of the brethren. Latter, I met with an informal committee answering to Mark E. Petersen, which had been set up to help members confronted with issues raised by fundamentalist Mormons (the Adam-God doctrine being one of the chief of these). The net result of my meetings with these people began to make me realize that Brother Petersen wasn’t acting out of ignorance of the facts regarding the Adam-God problem, and neither was Bro. McConkie.

I still wondered about the extent of President Kimball’s knowledge of the subject, however. I suspected that my letter had never reached him. In February 1981, I met with Michael Watson, the secretary to the First Presidency. He was surprisingly candid with me, revealing that my letter had been forwarded to Mark E. Petersen. Brother Watson showed me a memo written by Brother Petersen to the First Presidency with his recommendations as to how to respond to me. He informed them that the issues I had raised were real, that Brigham Young had indeed taught these things, but that they could not acknowledge this lest I would “trap them” into saying this therefore meant Brigham was a false prophet. He therefore recommended that I be given a very circuitous response, evading the issue, which he volunteered to write. I asked Brother Watson, as well as members of the committee I had previously met with, how this approach would help people like myself who knew better? Wasn’t there concern that some might be dismayed and disillusioned by their church leaders’ lack of candor? Their response was very similar to President Hinckley’s statement mentioned earlier about losing a few through excommunication: they said, in essence, “If a few people lose their testimonies over this, so be it; it’s better than letting the true facts be known, and dealing with the probable wider negative consequences to the mission of the church.

Acknowledgement: the above presentation follows closely after point #2 from this blog post in logic, content, and some verbiage which I borrowed directly.