Avraham is described in Parshas Toldos (26:5) as one who observed the Torah of Hashem. Chazal (Kiddushin 82a) explains that this passuk is teaching us that Avraham observed the entire Torah even before it was given. The Ramban in his commentary on this passuk elaborates on this statement of Chazal. Yaakov also observed the mitzvos prior to them being given but only did so in Eretz Yisroel. This was the justification for Yaakov marrying two sisters, and as such Rachel actually died as he returned to Eretz Yisroel. The Ramban adds that although mitzvos are binding outside of Eretz Yisroel, the primary place for mitzvah observance is in Eretz Yisroel. Thus, the voluntary observance of the avos was limited to when they were present in Eretz Yisroel.
This premise of the Ramban, that there is a fundamental distinction between mitzvos performed in Eretz Yisroel and those performed outside of Eretz Yisroel, appears difficult to understand. Agricultural mitzvos such as terumah, ma'asros, and shemitah are linked to the land and do not apply in Chutz La'aretz. Mitzvos which are chovas haguf, those performed with one's body, have to be observed outside of Eretz Yisroel and yet the Ramban understands them to be on a higher level if done in Eretz Yisroel. Why should mitzvos which are not connected to the agriculture of Eretz Yisroel still take on an additional dimension when done in Eretz Yisroel?
Chazal (Keilim, chapter 1) delineate the ten level of geographic kedusha that exists in the world. The place with the most intense kedusha is the Kodesh haKodoshim. Different areas of the Beis Hamikdash and Yerushalayim are each endowed with various degrees of kedusha. The tenth and final area mentioned is Eretz Yisroel. Each area has its own halachos that differentiates it from the other areas. The kedusha of Eretz Yisroel which separates it from the rest of the world is the fact that the korbanos of the omer and the shtei halechem offered on Pesach and Shavuos can only be brought from grain that was grown in Eretz Yisroel. Rather than the obvious halachik distinctions between Eretz Yisroel and Chutz La'aretz such as terumah, ma'asros, and shemitah, why do Chazal highlight the halachos that are related to korbanos?
The mefarshim explain that the theme of these mishnayos which differentiates between different levels of kedusha is the gradations of kedusha emanating from the Beis Hamikdash. Beginning with the Kodesh haKodoshim and ending with Eretz Yisroel, there are ten levels of kedushas ha'aretz. It would be irrelevant for the mishna to highlight the agricultural mitzvos that apply only in Eretz Yisroel as the mishna is not focusing on those distinctions.
The omer and the shtei halechem are korbanos that must come from an area endowed to some degree with kedushas ha'aretz. Eretz Yisroel has sufficient kedushas ha'aretz to enable these korbanos to be brought from grain grown in its borders.
Eretz Yisroel is distinct from Chutz La'aretz in two ways. It is agriculturally different which results in a practical difference concerning mitzvos pertaining to the land and it is also different in that it has kedushas ha'aretz which Chutz La'aretz does not. It is this second dimension of Eretz Yisroel that results in its unique status concerning all mitzvos. The primary location for the performance of all mitzvos is in the Beis Hamikdash, the place dedicated for avodas Hashem. The outermost precincts of the Beis Hamikdash end at the borders of Eretz Yisroel. Thus, the entire land is the primary location for mitzvah observance. Although the Torah clearly obligates us to fulfill mitzvos even in Chutz La'aretz, the Ramban understands this to mean that these mitzvos are still not at the level of mitzvos performed in Eretz Yisroel.
The avos who volunteered mitzvah observance only did so in Eretz Yisroel where the highest level of fulfillment of the mitzvos could be achieved.
This aspect of Eretz Yisroel as an extension of kedushas ha'aretz explains another halacha that does not apply in Chutz La'aretz. Chazal teach us that the declaration of Rosh Chodesh must be done by a beis din in Eretz Yisroel. The Rambam elaborates upon this theme by applying this even to our observance of Rosh Chodesh today. In the absence of the process of witnesses testifying that they saw the new moon and the subsequent declaration of Rosh Chodesh by beis din, Rosh Chodesh today is "declared" by the Jewish people observing it as Rosh Chodesh. The Rambam states that it is this observance-declaration of the Jewish community in Eretz Yisroel that determines the day of Rosh Chodesh which establishes Rosh Chodesh worldwide. Why is Eretz Yisroel so central to the observance of Rosh Chodesh, given that Rosh Chodesh has nothing to do with the agricultural uniqueness of Eretz Yisroel?
The declaration of Rosh Chodesh emanates from the Beis Hamikdash, as all Torah ultimately comes from the Beis Hamikdash which housed the aron and was the seat of the Sanhedrin. From Eretz Yisroel, the outermost area endowed with kedushas ha'aretz, goes forth the declaration of Rosh Chodesh. Whether by the formal announcement of beis din or the observance of the people, the new moon is sanctified in Eretz Yisroel. As we are about to observe Rosh Chodesh this coming week, we turn to Eretz Yisroel and realize its centrality in our lives. From the days of the avos until today, Eretz Yisroel remains the primary location for mitzvah observance. Even as we follow the commandment of the Torah to continue performing mitzvos in Chutz La'aretz, we look forward to the day when mitzvos will be performed in their complete glory in Eretz Yisroel blessed with the Beis Hamikdash rebuilt in its midst.