Short answer: Because the birth of a son reopens a long-standing, male-preference succession expectation that Viserys had already overturned by naming Rhaenyra heir — so many nobles view Aegon as the “natural” heir and see Rhaenyra’s position as abnormal or temporary. Viserys’s earlier break with tradition (naming a daughter) and then keeping Rhaenyra as heir after fathering a son shocks and splits the court, giving rival factions a rallying point and turning a dynastic question into a political contest that fractures the realm.
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