Rabbi Ehrman already beat me to writing up an amazing Maharal on last week's parsha that says that Avraham only achieved greatness in Eretz Yisrael. How can this be, asks R' Hartman in his footnotes? Avraham was 75 years old when he came to Eretz Yisrael. He already was a big tzadik, he already was being makareiv and megayeir people -- he had achieved all this starting from nothing while living in chutz la'aretz. Was this not greatness? Yet we see from the Maharal that Avraham's position as one of the Avos was not possible without Eretz Yisrael.
Why should this be the case? I would like to suggest that the Avos had dual role: the Avos were not simply great tzadikim, but were also (as the name suggests) the founding fathers of our nation. By definition, a nation must have a homeland. Founding fathers must, in fulfilling that role, help establish that homeland. George Washington could not be the founding father of the USA had he lived in England, even if he would have supported the Revolution more than any other patriot on Continental soil. Avraham Avinu might have been among the greatest of tzadikim, but without coming to Eretz Yisrael he could not have been the father of our nation.