To make them in line with the rarity and currency-type of the breed (direwolf) they are supposed to accompany. It makes no sense for the breed to be a pebble-item and the accompanying genes to be moonstones. All other gene and breed combinations (that are purchasable) are for matching currencies. Also making the genes moonstone completely undoes the attempt at moonstone-pebble balancing releasing the breed for pebbles was trying to achieve.