Genesis, in many ways, is the story of the Abrahamic Covenant being fulfilled:
The Lord said to Abram, “Go forth from your native land and from your father’s house to the land that I will show you.
I will make of you a great nation,
And I will bless you;
I will make your name great,
And you shall be a blessing.
I will bless those who bless you
And curse him that curses you;
And all the families of the earth
Shall bless themselves by you.”—Genesis 12:1-3 (NJPS)
The drama from the individual stories arises mostly from the conflict between this promise and a variety of threats to its fulfillment. Central to the movement of the story from one generation to the next is the question of which member of Abraham's family will be the inheritor of this covenant. We see several stories of close calls when some more powerful man threatens to father children with the wives of a patriarch. In fact, the first occurs immediately following the establishment of the covenant in Genesis 12:10-20 when Abraham and Sarai go down to Egypt because of a famine and pharaoh nearly takes her as a wife.
Meanwhile, each Patriarch has his own set of problems producing a legitimate heir. Abraham produces Ishmael and Issac, but only Issac is chosen. Issac produces Esau and Jacob, but only Jacob is chosen. Jacob intensifies the problem by producing 12 sons. Normally, the inheritance would pass to the oldest, but Jacob has no loyalty to that law and has clearly signaled his favor to Joseph. What's more, Joseph receives two dreams that indicate God favors him too. The second involved not only Joseph's brothers, but his parents as well:
He dreamed another dream and told it to his brothers, saying, “Look, I have had another dream: And this time, the sun, the moon, and eleven stars were bowing down to me.” And when he told it to his father and brothers, his father berated him. “What,” he said to him, “is this dream you have dreamed? Are we to come, I and your mother and your brothers, and bow low to you to the ground? So his brothers were wrought up at him, and his father kept the matter in mind.—Genesis 37:9-11 (NJPS)
Now we know the bigger story of jealousy, revenge and despair, but from Joseph's perspective, it must have been bewildering. Might not his father have changed his mind about giving his property and rights to an arrogant teenager who can't keep from boasting? Perhaps God intended to put Joseph through the trial his grandfather endured—being placed on the altar of blood sacrifice only to be rescued by an angel at the last moment? Whatever else might be true, Joseph had to assume the blessing and promise would pass to one of his brothers rather than to him.
For the next 13 or so years, Joseph was not able to move where he pleased but was held as a slave and a prisoner. But the situation changed after he was able to interpret Pharaoh's dreams:
Pharaoh said to Joseph, “I am Pharaoh; yet without you, no one shall lift up hand or foot in all the land of Egypt.” Pharaoh then gave Joseph the name Zaphenath-paneah; and he gave him for a wife Asenath daughter of Poti-phera, priest of On. Thus Joseph emerged in charge of the land of Egypt.—Joseph was thirty years old when he entered the service of Pharaoh king of Egypt.—Leaving Pharaoh’s presence, Joseph traveled through all the land of Egypt.—Genesis 41:44-46 (NJPS)
Two point in relation to the question:
Joseph had considerable freedom within the land of Egypt, but he seems to have exchanged his Hebrew identity with an Egyptian one. He was given and Egyptian name, family, and position. Contacting his father at this point, might have raised questions about his loyalty. More so, since his new position would have required him to be exceptionally busy.
Joseph could not know anything of the drama back home. As far as he was concerned, his father might have disowned him or established one of the other brothers as favorite. Maybe the sons killed their father too? Perhaps God fulfilled the dreams when Joseph became vizier of the land.
At any rate, when his brothers show up, Joseph has lived in Egypt more than half his life and while he recognizes them, they don't know him. He has become an Egyptian. (Even his burial would be an Egyptian one.) The tension in chapters 42-44 turns on Joseph not knowing if he can trust his brothers. Joseph seems especially concerned about his mother's son, Benjamin. The tension is released in one of the climaxes of the Bible:
Joseph could no longer control himself before all his attendants, and he cried out, “Have everyone withdraw from me!” So there was no one else about when Joseph made himself known to his brothers. His sobs were so loud that the Egyptians could hear, and so the news reached Pharaoh’s palace.
Joseph said to his brothers, “I am Joseph. Is my father still well?” But his brothers could not answer him, so dumfounded were they on account of him.
Then Joseph said to his brothers, “Come forward to me.” And when they came forward, he said, “I am your brother Joseph, he whom you sold into Egypt. Now, do not be distressed or reproach yourselves because you sold me hither; it was to save life that God sent me ahead of you. It is now two years that there has been famine in the land, and there are still five years to come in which there shall be no yield from tilling. God has sent me ahead of you to ensure your survival on earth, and to save your lives in an extraordinary deliverance. So, it was not you who sent me here, but God; and He has made me a father to Pharaoh, lord of all his household, and ruler over the whole land of Egypt.—Genesis 45:1-8 (NJPS)
Similarly, after Jacob died:
His brothers went to him themselves, flung themselves before him, and said, “We are prepared to be your slaves.” But Joseph said to them, “Have no fear! Am I a substitute for God? Besides, although you intended me harm, God intended it for good, so as to bring about the present result—the survival of many people. And so, fear not. I will sustain you and your children.” Thus he reassured them, speaking kindly to them.—Genesis 50:18-21 (NJPS)
While there's an issue of Joseph recalling the story in light of his current status (and an even bigger issue of Moses reinterpreting the story as well), we can see that Joseph saw the time apart from his family and homeland as a provision from God. Better than anyone, Joseph can see how the fate of God's promise to Abraham was achieved by God's grace and not by Joseph's efforts. In turn, Joseph can be gracious to his brothers who tried to destroy him.
Joseph could not know his status with Jacob and the other 11 brothers when he finally was given the freedom to contact his birth family. Simultaneously, he received an Egyptian identity. Reading between the lines, he neither expected nor desired to see them again. But when he discovered the Lord had used him to fulfill the covenant with his great-grandfather, Joseph was able to see the bigger picture and forgive his bothers.