Han Sen's day hadn't gone well. He beat Bai Wuchang, but the primary root king dragon was taken by another prince.
Han Sen wasn't strong enough to beat another King class prince and seven half-deified guards. Even if he could have won, Bai Wei didn't want to offend the prince. Thet relinquished the primary root.
Bai Wei selected a smaller king dragon to practice with. Han Sen thought doing that was meaningless. If they didn't claim a primary root king dragon, then there wouldn't be much king air for him to absorb. Refining Ghost Bone power would be faster and more efficient.
King Bai had one hundred children, but there were only thirty primary root king dragons. Claiming one of them without fighting was very rare. You had to have a lot of luck to secure one peacefully.