“And do not let the Land spit you out…” (18:28)
The Ramban says that all other lands are under the auspices of the spiritual forces in heaven, but Eretz Yisrael is like the palace of the King, under Hashem’s direct control. But then it is not the Land which is spitting out, it is the king who cannot abide the reprehensible acts in His Presence. The verse implies, however, that the Land itself cannot tolerate iniquity.
What is special about Eretz Yisrael that makes it so sensitive?
In Parshas Ki Savo we read, “And Hashem brought us to this place and He gave us this Land…” The Sifri says that in the merit of coming to “this place” He has given us this Land. That means that Eretz Yisrael is in the mnerit of the Bais Hamikdash. It is if the Torah is telling us that Hashem wants everyone to have a close connection with Him which would be most shown by serving in the Bais Hamikdash. But not all of Klal Yisrael can be in the Bais Hamikdash all the time.
So Hashem expanded the sanctity of the Bais Hamikdash into Eretz Yisrael so that we could all live around the Mikdash like we were encamped in the desert around the Mishkan. (In fact the way the tribes settled the land is very similar to the way they were encamped in the desert.) In other words, Eretz Yisrael is the extension of the Bais Hamikdash.
Perhaps this is hinted to in the story of Yaakov Avinu when he awoke on the Temple Mount. Hashen told him that the land that you rested your head upon will be given to your children. But he only rested his head on a small area. Rashi says that all of Eretz Yisrael folded itself underneath him. There in the place of the Bais Hamikdash all of Eretz Yisrael was given to Yaakov and it unfolded for his children when they came to inhabit it.
We are obligated to remove contamination from the Bais Hamikdash because of the sanctity of the place. As well, Hashem says that the extension of the Bais Hamikdash cannot abide contamination of the activities of the Canaanites. If we act that way we will be spit out.