Why were the Jews and the Christians kicked out of Hijaz? I understand the slight enmity of the Jews against the Muslims (at times, it was warranted), however, I'm still unsure as to why the Christians, in particular, were kicked out.

Reference from the Sahihs:

Sahih Muslim Book 019, Hadith Number 4366.

It has been narrated by 'Umar b. al-Khattib that he heard the Messenger of Allah (may peace be upon him) say: I will expel the Jews and Christians from the Arabian Peninsula and will not leave any but Muslim.

Sahih al-Bukhari 3152, Book 57, Hadith 60.

It has been narrated on the authority of Abu Huraira who said: We were (sitting) in the mosque when the Messenger of Allah (ﷺ) came to us and said: (Let us) go to the Jews. We went out with him until we came to them. The Messenger of Allah (ﷺ) stood up and called out to them (saying): O ye assembly of Jews, accept Islam (and) you will be safe. They said: Abu'l-Qasim, you have communicated (God's Message to us). The Messenger of Allah (ﷺ) said: I want this (i. e. you should admit that God's Message has been communicated to you), accept Islam and you would be safe. They said: Abu'l-Qisim, you have communicated (Allah's Message). The Messenger of Allah (ﷺ) said: I want this... - He said to them (the same words) the third time (and on getting the same reply) he added: You should know that the earth belongs to Allah and His Apostle, and I wish that I should expel you from this land. Those of you who have any property with them should sell it, otherwise they should know that the earth belongs to Allah and His Apostle (and they may have to go away leaving everything behind).

I also understand that they were not kicked out within the lifetime of Muhammad, but by Umar after the death of Muhammad. Although Muhammad commanded their removal.