I think it's to keep the power line clear, cuz they're trying to collect the devil's frag so naturally they need to converge them into only one vessel, which is whoever the king is.
Although the Prince is already a possessor, it doesn't mean it has the frag from its parents so until they die he can't inherit it. The Prince needs to have a child before he gets the inherited frag cuz the child would be too powerful(?).
So it's like, collect (the whole frag) then pass on, then collect again. I THINK, I REALLY USED MY BRAIN HERE AND THERE'S STILL SOME CONFUSION. THIS IS JUST MY UNDERSTANDING.
I think it’s because the transfer of the fragment to the crown prince happens when the king and queen die and if he has children after that, he can transfer the fragment when they’re born...so i think the thought process is that the king (prev crown prince) needs to monopolize the fragment for as long as he lives and dont let his kids have it after just having obtained it from the previous king. They make it so the only way he can pass it down is when he dies. That was my interpretation after re-reading that part a few times
King or queen hold the frag. By tradition they will have their baby before inherit the frag from their parent. So t
Current prince do not have frag. Prince must have his heir before king n queen die so that the grandchild is fragment-free. Once king is dying, the prince will stay at his presence and inherit the frags-the first option mentioned by arhad.
If he does not have children its save to say when he dies, who ever is nearest to him will get the frag? If nobody is nearest the frag will dissipate and returned back to the world. No owner.

Devil’s frag -> in possessor’s heart
Possessor - receives devil frag benefits
Co-possessor - blood relative of possessor that shares these benefits
There can be only one possessor but unlimited # of co-possessors.
Possessor - most powerful.
Co-possessor - more powerful the closer in blood to possessor
3 ways the devil frag is transferred:
1. Death of possessor
- frag goes to co-possessor closest in blood
- if no co-possessor, devil’s frag goes back into the world
2. Extortion of frag (by crushing possessor’s heart)
- possessor can’t steal frag from co-possessor because co-possessors don’t have frags since they’re just sharing from possessors
- co-possessors can steal frags from possessors while maintaining co-possessorship (they become BOTH possessor & co-possessor)
- if frag is stolen from possessor, the blood relatives (co-possessors) die
3. Pregnancy
- possessor bears a child -> baby becomes possessor
- that’s why before a Bahamut prince inherits crown, he must bear a son & daughter before king & queen die so babies wont inherit the frag (since king/queen are the current possessors, babies cant be possessor if parents arent possessors YET)
Feel free to correct me or add on :’))