It has everything to do with what "The Ultimate Reality" means. This is not aimply an arbitrary string of words, it an identifier!
How do you identify The Ultimate Reality?
Well, for it to be what it is, it must be ONE. The Ultimate Reality, by definition must be 1. Both the word "The" and the word "Ultimate" solidify this.
The Ultimate Reality must be perfect and complete. It must account for all other realities. If this wasn't the case, then a reality could exist independent of it, and if a reality existed independent of it, how could The Ultimate Reality be all encompasing? It must always be real, ultimately real. For that reason, there can be no reality apart from it. Reality apart from The Ultimate Reality is nonexistence. That iis not reality, it is the absence of reality.
Time as a reality can only exist by The Ultimate Reality. As this is the case, time can have no dominion over The Ultimate Reality. The Ultimate Reality cannot change, because if it changed, that would mean that it is subject to causal forces. If this was the case, causality and time would be greater realities. The Ultimate Reality can not be subject to another reality. If the entire universe acted in unison, it could not effect The Ultimate Reality. The universe is constantly changing, time has dominion in the universe. God is not effected by these things ,but God holds all of it in eternity, a timeless eternity. A perfection of that which is.
So if The Ultimate Reality could be replaced, changed, effected, it wouldn't truly be The Ultimate Reality. It has to be perfect. Complete. All encompassing. Unique. Singular.
How do I know? It has been revealed to me, and it can be revealed to you too if you ponder on that name.
The Ultimate Reality.
What does that mean? Meditate on it. You'll see I am not simply making this up. It's innate to what it is.