“I will multiply your seed as the stars of the sky, and will give to your seed all these lands. In your seed will all the nations of the earth be blessed, because Avraham listened to my voice, and kept my requirements (מִשְׁמַרְתִּי), my commandments (מִצְוֺתַי), my statutes (חֻקּוֹתַי), and my laws (וְתוֹרֹתָי).” (Genesis 26:4-5) According to the most straightforward interpretation of this verse, God promises to multiply Yitzchak’s descendents because Avraham had faith in the One True God (וַיִּשְׁמֹר מִשְׁמַרְתִּי); left his birth place, sacrificed Yitzchak, and banished Hagar and Yishmael (מִצְוֺתַי); followed the ways of God’s behavior — Gracious, Merciful, Just, and Lawful — and commanded his children to do the same (חֻקּוֹתַי); engaged in bris milah and the seven Noahide Laws (וְתוֹרֹתָי). (Ramban ad loc.)
However, according to Chazal, this pasuk can be taken as an indication that Avraham kept all of the halachos of the Torah as we keep them now. (See Genesis Rabbah 79:6 and 94:3.) Avraham divined these halachos through ruach ha-kodesh, but was never commanded to keep them by God. Avraham passed down this knowledge to his children and they to theirs, and so kept all of the Torah.
Because Avraham was never commanded to keep these mitzvos, he and his children only kept them in Eretz Yisrael. For instance, Yaakov married Rachel and Leah outside of Eretz Yisrael. Moreover, the timing of Rachel’s death was fortuitous, so to speak — she died just as Yaakov entered Eretz Yisrael such that Yaakov would not dwell in Eretz Yisrael while married to two sisters. (Ramban, Leviticus 18:25)
God distinguishes the Jewish people by prohibiting idolatry; He reserves them for Himself. To concrecretize and consecrate this relationship, He gives them His land. They can then achieve the sanctity of nationhood by observing His commandments there. ibid. However, God only provides His commandments to a nation with a homeland. Avraham, Yitzchak and Yaakov did not constitute a nation; they could therefore not have a homeland. Avraham, Yizchak and Yaakov, therefore, could never achieve the sanctity that comes from the commandments of the Torah. When they lived in what would become Eretz Yisrael, they could approximate the sanctity of nationhood. (Ramban compares their observance to the Halachic category of אינה מצווה ועושה.)
Outside Eretz Yisrael, there was no point in their shemiras ha-mitzvos. Indeed, we only keep the mitzvos outside Eretz Yisrael because our stature as a nation connects us to our home, to which we will eventually return. The Avos did not enjoy the stature of nationhood; they were completely cut off from the homeland.