Why yuhon never became king

I'm thinking that rather than the birth of princess yona being the reason il became king. It was because either the previous king gave him the ultimatum that if he married yonhi he couldn't ascend because all of his decendanents would have short lifespans. Or yuhon decided that il would take the throne so that the royal bloodline wouldn't have short lifespans. I think if this is the real reason then the second option is probably better because then soowon won't be able to cast blame on someone else. However this still doesn't answer how yuhon died.