>>279933891
The Dragon God probably looks for humans who are exceptionally talented in a bloodline's specialized magic if it needs a new King for that bloodline. I think the manga implies that how strong of a bloodline member you'll become depends on your aptitude for that specific bloodline, and that not just anyone qualifies for the position of King in DG's eyes.
I'm also guessing that instant maturation when joining a bloodline is a sign that you're King material for that bloodline