It was revealed to David O. McKay that Jesus Christ Himself didn't want the blacks to have the priesthood during David O. McKay's Presidency. That tells me that Jesus Christ Himself told Brigham Young NOT to give the blacks the priesthood at that point in time, which Brigham Young obeyed, as Brigham was obligated to do.
The timing of all of this, inside the LDS Church and outside the LDS Church, is the Lord's timing. He has control over it all. The Lord knows very well about these new policy changes in regards to same-sex marriage, because He had a hand in writing them and He could have vetoed them at any point along the way.
The Lord was also very much aware that the Blacks weren't given the Priesthood during and after Brigham Young's Presidency, because the Lord is the one who set that policy, and the Lord could have vetoed that policy at any point along the way. Eventually the Lord did veto that policy and gave additional revelations explaining why He wanted the policy to now be changed.
Clearly, the reasons why the Lord refused to give the Blacks the priesthood during Brigham Young's time were in fact different than what the Latter-day Saints of the era were surmising or speculating. That doesn't change the fact that it was the Lord's will that the Blacks not have the priesthood during that point in time.
The Lord knows exactly what He is doing, and the Lord is a lot smarter than the rest of us. The timing of all of this is His!