The United States government commissioned 122 Vaults to be built by Vault-Tec before The Great War took place. This should have been a clue that both the government and the Vault-Tec weren't worried about protecting citizens. So few Vaults were never enough to house the population of the US at that time, which was over 400 million. However, this information comes from The Fallout Bible, which isn't considered to be canon.