>>17843849 (OP)Because nobility was ordained by God and mingling with lesser bloodlines would weaken that mandate. The political analysis is that you weaken the entire concept of nobility if you start including non nobles or weaker houses among your dynasty.
Even in our time it’s unlikely the remaining royal houses will survive a generation beyond us because they all started marrying commoners at which point the illusion falls apart and they just become weird celebrities and nepo babies which the same people who follow royals hate.