Rashi at the beginning of Parshat Behar asks the famous question:
מה ענין שמיטה אצל הר סיני?””
“What special relevance does the subject of Shemittah have with Har Sinai?”
Rashi continues to explain that since all the Mitzvot were given on Har Sinai, why does the Torah specifically mention that fact only regarding the Mitzvah of Shemittah? Rashi’s question is based on the premise that all the Mitzvot were given on Har Sinai, something which is not so clear from the simple understanding of Chumash.
Indeed, many other mepharshim, based on the straightforward understanding of Vayikra, Chapter 1, Verse 1, understood that the entire book of Vayikra was transmitted to Moshe in the Ohel Moed and not on Har Sinai. They therefore ask not why the Torah mentions Har Sinai in relation to the Mitzvah of Shemittah, rather why are the Parshiyot of Behar and Bechukotai, which deal with the laws of Shemittah, Yovel, Loans, Eved Ivri and various other Mitzvot regarding Bein Adam le’Chavero, “added on” at the end of Sefer Vayikra, if they were transmitted to Moshe many months earlier at Har Sinai?
The Ibn Ezra, employing the dictum “אין מוקדם ומאוחר בתורה” – “In the Torah, chronological order is not followed”, says that although these Parshiot were given previously at Har Sinai, they are placed at the end of Vayikra since, as is clearly apparent from the verses in the covenant in Parshat Bechukotai, they are fundamental conditions and prerequisites for the continued prosperity and longevity of Am Yisrael in Eretz Yisrael.
“Then, the land will be appeased regarding its sabbaticals. During all the days that it remains desolate while you are in the land of your enemies, the Land will rest and thus appease its sabbaticals.
It will rest during all the days that it remains desolate, whatever it had not rested on your sabbaticals, when you lived upon it.
[For] the Land will be bereft of them, appeasing its sabbaticals when it had been desolate of them, and they will gain appeasement for their iniquity. This was all in retribution for their having despised My ordinances and in retribution for their having rejected My statutes.”
Since the Torah, at the end of Sefer Vayikra is about to begin Sefer Bamidbar, which is essentially the preparation for the journey and entering into Eretz Yisrael, it therefore “cut and pasted” these Mitzvot and the covenant exactly here, although it had all been said and done previously on Har Sinai.
Other Mepharshim explain that the relevance of these Mitzvot to the end of Sefer Vayikra, also named “Torat Kohanim” because of the concentration of Kohanim, Mishkan and Korbanot related laws, is the connection of these laws to the Kohanim.
The Ramban, who traditionally refrains from using the solution of “אין מוקדם ומאוחר בתורה” , explains that these last two Parshiyot of Vayikra are in chronological order. They are placed here because they were told over to Bnei Yisrael at this time after the Mishkan had been built and the laws of Sefer Vayikra, Torat Kohanim had been concluded. They were given over to Moshe Rabeinu on Har Sinai after Chet Ha’egel and that’s why Har Sinai is mentioned here since Moshe is now relating to Bnei Yisrael these mitzvoth that were told to him then.
Perhaps another explanation could be suggested for the apparent misplacement of these two concluding Parshiyot.
As mentioned, Sefer Vayikra predominantly deals with the laws of the Mishkan, Korbanot and the Kohanim. Observance of these laws is what guarantees the purpose of the Mishkan being fulfilled. At the outset, when the Mishkan was commanded in Parshat Teruma, the Torah states: 
ועשו לי מקדש ושכנתי בתוכם””
“And they shall make Me a Mishkan and I will dwell in their midst.”
Repeated again at the end pf Parshat Tetzaveh:
“ושכנתי בתוך בני ישראל והייתי להם לאלקים. וידעו כי אני ה’ אלקיהם אשר הוצאתי אתם מארץ מצרים לשכני בתוכם אני ה’ אלקיהם”
“I will dwell in the midst of the children of Yisrael and I will be their G-d.”
They will know that I, the Lord, am their G-d, Who brought them out of the land of Egypt in order that I may dwell in their midst; I am the Lord, their G-d.
The objective of the Mishkan is for there to be a place where the Shechina, as if, dwells – a physical reality where the imminent and transcendental exist together. The same idea holds true for the Beit Hamikdash, Beit Hashem, that we long for. Had Sefer Vayikra ended at the end of Parshat Emor, it would seem that that was the ultimate goal of our covenant with Hashem.
However, the concluding verses of the Berachot in Parshat Bechukotai state the following:
“ונתתי משכני בתוככם ולא תגעל נפשי אתכם. והתהלכתי בתוככם והייתי לכם לאלקים ואתם תהיו לי לעם”.
“And I will place My dwelling in your midst…
I will walk among you and be your G-d, and you will be My people.”
The ultimate purpose of the Brit between Hashem and Am Yisrael is not merely the creation of a specific place for the Shechina to dwell. Rather it is to create a reality where the Shechina permeates our whole existence, all over the land. The purpose of the brit and all the commandments that come with it is not to create the greatest of sanctuaries, rather the purpose of the holy sanctuary, is to create a land imbued with G-dly Ideals.
Therefore, at the end of all the parshiyot dealing with the Mishkan the Torah purposely comes back to those Mitzvot which are essential and crucial for the creation of a just and moral society in order to reiterate and remind us of the initial words of Hashem to us on Har Sinai:
“ואתם תהיו לי ממלכת כהנים וגוי קדוש”
“And you shall be to Me a kingdom of princes and a holy nation”
Says the Ba’al Haturim on that Pasuk:
“אילו זכו ישראל היו כולם כהנים גדולים. ולעתיד לבא תחזור להם שנאמר (ישעיה סא ו) ואתם כהני ה’ תקראו.”
“If they had merited, they would all have been High Priests. And indeed, in the future to come, it shall come to be again to them, as it is written, ‘And you, the Priests of Hashem you shall be called.”
 Rashi, Vayikra 25:1.
 See Eliyahu Mizrahi and Gur Aryeh on Rashi ibid.
 Ibn Ezra, Rashbam, Bechor Shor, Chizkuni and others.
 Vayikra 26:34,35, 43.
 Bechor Shor, Chizkuni.
 בכור שור ויקרא פרק כה פסוק א
בהר סיני. כל הפרשיות כולן של מעלה מראש הספר עד כאן נאמרו מאהל מועד, וכולה שייכי באהל מועד: הקרבנות הטהורות והטמאות שצריכין ליזהר מליכנס ומלאכול בקודש בטומאה, כגון זבים ומצורעים ובעלי קריים ויולדות ומלואים וכל העניין, אבל אילו שני הפרשיות בהר סיני ואם בחקותי לא שייכי כל כך לאהל מועד ולא לכהנים ונאמרו בהר סיני. ומיהו משום דשייכי קצת שמדברות בשביעית ויובל וערכין דשייכי בכהנים, שהכהנים מקדשים השנים, וביובל כתיב: “והעברת שופר תרועה”1, וכתיב: “ובני אהרון הכהנים יתקעו בחצוצרות”. וגם מדברים בחרמים שהם לכהנים, ובערכין שהכהנים מעריכין, נצטרפו הכא בתורת כהנים. ומשום דלא תיסק אדעתין דנאמרו באהל מועד, כתב דבהר סיני נאמרו.
 Ramban Vayikra 25:1.
 According to the Ramban it’s not clear why the Torah wouldn’t say that Moshe told the people the Mitzvot he was told on Har Sinai instead of simply saying that Hashem spoke to him, which means -now.
 Shmot 25:8.
 Ibid 29:45,46.
 Likutei Sichot, Schneerson, Vol 1, Terumah .
 Vayikra 26:11,12.
 Shmot 20;6.