if netilat yadayim is done in the morning to cleanse ourselves from any tumah while being asleep, then why not also on yom kippur?

Shalom, Thank you for your question. In fact you are correct, and we do wash our hands on Yom Kippur morning to remove the impurity of a night's sleep. However, as there is some Rabbinical debate as to whether only the fingers are impure, or the whole hand, we are strict to wash only the fingers, so as not to break the law forbidding unnecessary washing on the holy day. So, upon awakening on Yom Kippur morning on should wash their fingers three times (as on a normal morning) – but not more than the fingers, that is not the palm of the hand. Blessings.