Because he thought he wouldn't need to use them. He expected Ukraine to accept the inevitable and not provide meaningful resistance. He almost got away with it, too. Had the Russian army launched its missile stockpiles at Ukrainian powerplants on day 1, it would've done enough damage to overwhelm the country. However, he declared that "Ukrainians were not our enemies, only the Ukrainian leadership and specific armed nationalist groups were", so he couldn't have attacked the country's civilian infrastructure until after the citizens of Ukraine decided that Russia was their enemy.