Sorry, but yes, we DO know why.
It is because the blacks were not valient in the pre-existence, right?
LDS "Apostle" Bruce R. McConkie: "Those who were less valiant in pre-existence and who thereby had certain spiritual restrictions imposed upon them during mortality are known to us as the Negroes. Such spirits are sent to earth through the lineage of Cain, the mark put upon him for his rebellion against God and his murder of Abel being a black skin" (Mormon Doctrine, by Bruce R. McConkie, Bookcraft, 1958 edition, pp. 476-477; in second edition, 1966, p. 527; removed from 1979 edition).
"Negroes in this life are denied the priesthood; under no circumstances can they hold this delegation of authority from the Almighty. The gospel message of salvation is not carried affirmatively to them . . ."
"Negroes are not equal with other races where the receipt of certain spiritual blessings are concerned . . . " (Mormon Doctrine, by Bruce McConkie, Bookcraft, 1958 edition, p. 477; changed in later editions).
There is some history with McConkie's book. When he published it in 1966 he was not an Apostles and it was never considered "the word of God". Many of the Quorum of the Twelve were not happy about it precisely because of this reason, it tried to set doctrine in stone when
9 We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God.
Paul wrote that we see through a glass darkly, he knew in part. We simply do not know at this time God's reasons for why some are born in a jungle and another in Trump Tower.
Many in the Church disagreed with McConkie's reasoning and how e and others came to their conclusion. Joseph Smith disagreed and Brigham Young who instituted the ban disagreed. Young said one day they would receive the priesthood, all of the Temple blessings and more.
I feel McConkie based his reasoning on some faulty assumptions. This will be hard for you to understand because you don't believe in a pre-earth life and you do not accept the passages I'll be quoting from as scripture, but I want you to understand why I feel McConkie was wrong.
1, His reasoning was drawn from what happens at death, we are judged by our actions here. If we accept Jesus here and follow his word then in the next life we gain a greater reward. McConkie applied that doctrine to the pre-existence. If we were valiant in our pre-earth life then we got a great reward here.
2,He also took two passages from the Book of Abraham and tried to make them fit. Abraham sees the pre-existenceing spirits and learns " if there be two spirits, and one shall be more intelligent than the other" so we are not all the same, we each have varying amounts of abilities. He is also shown "and among all these there were many of the noble and great ones" who would be made the rulers. He reasoned that some of us are just greater spirits than others, which is true to a point.
3, Also Abraham writes about the priest of Egypt who McConkie believed had the mark of Cain and says and those who came through the family of Ham.
26 Pharaoh, being a righteous man, established his kingdom and judged his people wisely and justly all his days, seeking earnestly to imitate that order established by the fathers in the first generations, in the days of the first patriarchal reign, even in the reign of Adam, and also of Noah, his father, who blessed him with the blessings of the earth, and with the blessings of wisdom, but cursed him as pertaining to the Priesthood.
27 Now, Pharaoh being of that lineage by which he could not have the right of Priesthood, notwithstanding the Pharaohs would fain claim it from Noah, through Ham, therefore my father was led away by their idolatry;
So from this McConkie and others drew the conclusions that all blacks could not receive the priesthood and it was tied to their pre-earth life.
Now where did he go wrong.
1, There is no scripture supporting the idea that some were more valiant, instead the Lord taught Joseph Smith;
"Man was also in the beginning with God. Intelligence, or the light of truth, was not created or made, neither indeed can be....Every spirit of man was innocent in the beginning..." D&C 93
2, Yes I believe we all have varying amounts of abilities and weaknesses however we attain perfection through Christ. Paul prayed about the thorn in his side and the Lord answered;
And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. 2 Cor 12
In the Book of Mormon it says
"And if men come unto me I will show unto them their weakness. I give unto men weakness that they may be humble; and my grace is sufficient for all men that humble themselves before me; for if they humble themselves before me, and have faith in me, then will I make weak things become strong unto them." Ether 12
Through the grace of Christ all of our weakness even those we brought with us into this life will be made strong and we will attain perfection.
Plus many of the great and nobles ones who became his rulers are not righteous men, they become corrupt and they will be held accountable. Many more very unassuming people are righteous followers of Christ. Who we were in our pre-earth life does not determine what our final judgement will be.
3, I feel like McConkie didn't read the whole chapter of Abraham 1 or it went right over his head, like so many of us we have preconceived ideas and he saw only what he wanted to see. Abraham was talking about the patriarchal order of the priesthood.
"I sought for the blessings of the fathers, and the right where unto I should be ordained to administer the same; having been myself a follower of righteousness, ....I became a rightful heir, a High Priest, holding the right belonging to the fathers....even the right of the firstborn, or the first man, who is Adam, or first father, through the fathers unto me."
The priesthood was passed through a patriarchal order. As I said earlier in the beginning all of Adam's sons would have been given the priesthood and then passed it down through their sons.
Cain lost his priesthood when he committed murder so he had no priesthood to pass on. When we get to Noah he only passes the priesthood on to Shem, both Ham and Japheth are band from the priesthood.
In chapter 1 of Abraham he explains that some of his own fathers had turned to idolatry, see Joshua 24. So even though they held a right to the receive the priesthood they were not given it. He receives his own priesthood through Shem/Melchizedek . Technically one of Cain's sons could have gone to Adam and asked for the priesthood as Abraham did to Shem.
Now you will note that in 1979 what McConkie had written was removed from his book that's because now as an Apostle he disavowed it;
"There are statements in our literature by the early Brethren which we have interpreted to mean that the Negroes would not receive the priesthood in mortality. I have said the same things, and people write me letters and say, “You said such and such, and how is it now that we do such and such?” And all I can say to that is that it is time disbelieving people repented and got in line and believed in a living, modern prophet. Forget everything that I have said, or what President Brigham Young or President George Q. Cannon or whomsoever has said in days past that is contrary to the present revelation. We spoke with a limited understanding and without the light and knowledge that now has come into the world.
We get our truth and our light line upon line and precept upon precept. We have now had added a new flood of intelligence and light on this particular subject, and it erases all the darkness and all the views and all the thoughts of the past. They don’t matter any more.
It doesn’t make a particle of difference what anybody ever said about the Negro matter before the first day of June of this year, 1978. It is a new day and a new arrangement, and the Lord has now given the revelation that sheds light out into the world on this subject. As to any slivers of light or any particles of darkness of the past, we forget about them. We now do what meridian Israel did when the Lord said the gospel should go to the Gentiles. We forget all the statements that limited the gospel to the house of Israel, and we start going to the Gentiles...."
It's okay for a man of God to come to a greater understanding, that's what revelation is all about.