>>35297960Because it appears at the literal ass end of BW, so to compensate they made it evolve very late (around the time you'd be expected to fight N and Ghetsis, but in reality you probably won't achieve this until AFTER the main game) rather than have it immediately evolve upon catching and leveling it, which would thus render the base form pointless because it didn't appear sooner.
Several Pokemon are like this in Gen 5 along with Braivary and its counterpart, namely Hydreigon, Volcarona, Mienshao and Bisharp.
