Time and space. The conditions were not ripe for the revelation to come until Pres. Kimball’s administration. As I said earlier Pres. McKay was somewhat distraught that he couldn’t get an affirmative answer. If he was just winging it and was simply a man doing the best he could doesn’t it seem obvious to you that he would have ‘manufactured’ a revelation? He was taking a certain amount of flack from some brethren as a result of not ‘caving’. He believed that he could not go against the will of the Lord.
From all accounts Pres. McKay was a kind and gentle soul. I’m fairly confident that he personally was on the same page as Hugh B. Brown and others but at the same time he knew he could not disobey the will of the Lord AT THAT TIME.
I suppose that’s mainly my opinion. Are you aware of any church doctrine that explicitly says God is obligated to answer any and all questions at any given moment in time when a prophet might ‘demand’ it? One thing to remember is that a prophet may beseech an answer from the Lord but the answer might be ‘wait’ or ‘no’.
A question for you and the other folks here.
Do you believe in the divine callings of other prophets such as Isaiah, Moses, Abraham, and other Old Testament prophets? And further do you believe that Jesus Christ was and is the ‘chosen one’, the Son of God?
Why does this matter? Well, I think you can readily figure that one out.

Regards,
MG