Or, much simpler: narrative convenience? Or Narrative Causality. Things happen because the plot says they should.
It's the same reason why the third son always inherits the kingdom, or why the swineherd always beats the Unbeatable Bad Guy(TM): Because I Said So(C).
The Italians have a wonderful proverb for stories like this: se non è vero, è molto ben trovato. Even if it's not true, it's a very good fabrication (or story).