Why does the Pasuk not begin with "Dabru el B'nei Yisrael" (as it does bew 15:2, in the Parshah of Zavim)?
Ramban #1: Because it is self-understood that they would pass the laws of Tzara'as Batim on to Yisrael. 1
Ramban #2: The Torah is currently concerned that Moshe and Aharon should learn the laws of Tzara'as and teach them to the Kohanim. Moshe intended to warn Yisrael later 2 before they entered the Land. 3
Bearing in mind that the Pasuk inserted "b'Veis Eretz Achuzaschem", which applied to all the people (Ramban). Refer also to 13:1:1:1*. See also Ba'al ha'Turim, 15:2.
This implies that the laws of Tzara'as would not come into effect before they entered Eretz Yisrael (See R. Chavel's footnotes and Yoma 14a, cited by the Torah Temimah).
Why does the Torah write "v'Nasati Nega Tzara'as", and not simply "Ki Sih'yeh Nega Tzara'as"?
Rashi: In order to introduce the good news that Tzara'as on the houses in Eretz Cana'an would result in having to demolish them - and find the treasure that the Cana'anim had hidden there.
Ramban: It is to stress that Tzara'as Batim, like Tzara'as Begadim, belongs entirely to the realm of the supernatural. 1
Moshav Zekenim: The verse alludes to Bayis Rishon and Bayis Sheni. "Asher Lo ha'Bayis" is Hashem. The Kohen is Yirmeyah (but below, it refers to Hashem). "K'Nega" is idolatry
Ramban (13:47): When Yisrael were on a high level the spirit of Hashem rested with them constantly, to ensure that their bodies, their clothes and their houses remained in good shape. That is why, when someone sinned, he was stricken with one of the categories of Tzara'as to demonstrate that Hashem had departed from him, as the Torah indicates here (that it is a stroke delivered by Hashem on that room). And it is by the same token that Tzara'as Batim is confined to Eretz Yisrael - because it is Hashem's chosen country in which His Shechinah rests (see Ramban, who elaborates further).
Why does Tzara'as not occur nowadays?
For which sin do Nig'ei Batim atone?
Rashi (in Succah, 56b): It atones for stinginess. 1
Targum Yonasan: For theft. 2
QUESTIONS ON RASHI
Rashi writes that Tzara'as on a house was good tidings. The Mishnah (12:6) says 'woe to a Rasha, and woe to his neighbor'!
Perhaps in some cases it was good tidings, and at other times, 'woe to a Rasha, and woe to his neighbor.' (PF)
Rashi writes that Tzara'as on a house was good tidings. What is good about Tzara'as on a garment?
Moshav Zekenim citing the Rosh, Hadar Zekenim: Hashem commanded to destroy things used to serve idolatry. People did not always know that garments were used for idolatry, so Hashem would send a Nega. 1
Moshav Zekenim said that Hashem strikes property first, so the sinner will repent before his body is stricken. A house is a greater loss than a garment, and it is a great hardship to clear out and leave one's house (especially when it is cold)! (PF)
This explains Nega'im only in garments that used to belongs to the Nochrim! (PF) (What was the basis of the question? Perhaps we expound that Tzara'as on a house was good tidings because it says "v'Nasati", an expression of a gift!)