Because that is the dragon gimmik. Have you not noticed this yet? o. They all have seemingly exclusive energy mixes in place of a dragon energy cost. And at least it has an all colorless attack.
However, in most cases, the energy costs have made some sort of sense.
Rayquaza has always had LR energy costs. Virtually every one of his printings has had an attack that cost LR.
Garchomp is a Land Shark. Ground and Water. Kinda makes sense.
Hydreigon is a little bit weird. But at least they kept the Dark part. Psychic is the antithesis of Dark, and the two are generally paired together in Pokemon lore.
Dragonite has had energy costs of both Grass and Lightning in the past.
Same for Salamence and Water/Fire.
Latias is a red Psychic Dragon. So the Fire/Psychic kinda makes sense there. Latios has similar justifications for Water/Psychic.
Haxorus as Steel/Fighting. In combination with his design, that kinda makes sense.
Druddigon is red and blue. How could they resist giving him Fire/Water costs?
Giratina is just about the only one that I know of that has truly weird energy costs. But, then again, maybe not. Both Grass and Psychic have been used for Poison in the past. I know Giratina isn't a Poison type, but that might be what they're thinking for that.
As far as Kyurem, that one seriously makes no sense. If anything, I might have given him Water/Lightning b/c of that yellow stripe on his head.