Why is a sacrificial animal (hady) offered in Qiran and Tamattu' Hajj, but not in Ifrad?

Slaughtering the Hady (sacrificial animal) is obligatory for the Mutamatti' (one performing Umrah then Hajj) and the Qaarin (one combining Umrah and Hajj), as a thanksgiving to Allah for allowing them to combine two rituals in one journey. As for the Mufrid (one performing Hajj only), the Hady is not obligatory for him, but it is commendable for him to offer it voluntarily.