“G-d said to Avram: ‘Leave your country, your birthplace, and your father’s house, unto the land that I will show you.” Avraham was Seventy five years of age when G-d appeared to him, giving him this command.
Interestingly, other than mentioning his birth, along with the birth of many otherwise not Biblically noteworthy members of Terech’s (Avraham’s father) family, this is the first we hear of Avraham in the Bible. We are told nothing at all about this man to whom G-d sees fit to give personal commands, nothing about how he came to recognize G-d, nothing about how he devoted himself to, and succeeded in, spreading the knowledge of G-d in the world and nothing about how he was thrown in a fiery furnace in the process (he escaped miraculously!) The written Torah gives no account of the accomplishments for which (presumably), G-d sees fit to establish this relationship with Avraham, not even a brief one line intro similar to the one with which the Bible introduces Noach (source).
The answer in short: Avraham becomes the first Jew and the beginning of the Jewish nation at this very commandment. And the (written) Torah is interested only in recounting Avraham as founder of the Jewish nation.
The explanation: all of Avrahams prior accomplishments –his recognition of G-d, his self sacrificing dedication to making him known and his character refinements– were all of his own initiative and of his own doing. And therefore, no matter how high he reached, relative to his fellow man, his accomplishments could never break out of the human qualitative limitations to which he himself was subject. The pinnacle of mankind though he was, he was not yet a foundation for the Jewish nation.
With the commandment, this all changed. G-d, after seeing what Avraham had managed, basically said, “O.K. great. Now leave everything behind and go to the land which I shall show you.”
At that point, and precisely at that point, Avraham’s divine service, now initiated and directed by G-d himself, was freed of the human limitations to which it had hitherto been subject. Avraham was uplifted beyond himself and the rest of mankind to act as founder to the Jewish people.