The "Why isn't ___ more popular" question is almost always answered by lack of demand.
I think I would turn the question around.
What is the use case for a 6x45? What does the extra 0.019" do that 223/556 doesn't.
It was an incremental change from 223 when it was wildcatted in 1965.
It doesn't have enough powder capacity to handle the heavier .243" bullets that have the best BC's for caliber now.
Wildcat brass and forming.
No major factory ammo support. Black hills used to make it, but not currently.
It is able to be used in jurisdictions where the minimum hunting diameter is more than .224"
To get to the "ideal" place where you have enough powder capacity to push the bullets with the best BC, we are talking about calibers that are too long for an AR-15 action, like the 6BR family, 6 Dasher, 6x47, etc.
The 6x45 is simply another wildcat that falls through the cracks. It doesn't have enough performance upgrade to differentiate from the standard caliber for AR-15's, but there are better options if a bolt action is chosen.