The most likely answer is marketing for Kyurem vs the Sword of Justice. Lore-wise, Keldeo's Resolute Forme was attained when it was strong enough to become one of the Swords of Justice, so the developers needed a relevant way to make it obtainable.
If you're less of a cynic than I am, you could say that the developers wanted to give attention to a potentially overlooked Mythical Pokemon by giving it a cool new form for players to use in battle.
