Among the central questions that have been addressed concerning the story of Yosef and his brothers is why Yosef had not contacted his family upon rising to power in Egypt. Even assuming he had no possibility of contacting his family while working for Potifar and certainly during his years in the Egyptian prison, once he was appointed viceroy he presumably had the means of sending a message to his grieving father to inform him that he is alive and well. (The Ramban famously addresses this question in his commentary to Parashat Miketz – 42:21.)
One of the lesser-known answers to this question was offered by Rabbenu Yehuda Ha-chasid, cited in Rav Menachem Kasher's Torah Sheleima (chapter 44, note 74): "Had he done so, all his brothers would flee – this one to the east, this one to the west – from shame. Instead, he took them with words, slowly and gently, so that they would not feel ashamed, and his intentions were good." According to this explanation, if Yosef had contacted his family earlier and told them that he was alive and in power in Egypt, they would have been unable to bear the humiliation, and they would have likely fled from Yaakov and from one another. For the sake of the family's unity, Yosef refrained from contacting his family and revealing his identity until this point, at the beginning of Parashat Vayigash.
It is unclear, however, how Yosef spared his brothers humiliation by not revealing his identity until after framing Binyamin with theft and hearing Yehuda's plea to allow him to return to Canaan. If anything, this process only intensified their shame. The Beit Ha-levi famously noted the sense of humiliation the brothers experienced when Yosef revealed himself and exclaimed, "I am Yosef – is my father still alive?" (45:3). Yehuda had just insisted that Binyamin be allowed to return home because their father would otherwise die from anguish (44:31); Yosef now turns to his brothers and rhetorically asks, "Is my father still alive?!" As if to say, "Why did you not think of this concern twenty-two years ago, when you sold me to slavery and subjected my father to two decades of anguish?" It is hard to imagine that the shame and humiliation experienced by the brothers at that moment was less than what they would have felt had Yosef notified his family of his whereabouts immediately upon being named Egyptian viceroy.
We might suggest a different approach, that Yosef's brothers would have indeed scattered "this one to the east, this one to the west" had he sent a message earlier, only for the exact opposite reason. Yosef likely assumed that the brothers felt no remorse over what they had done, and all throughout stood steadfastly by their conviction that he had to be eliminated from the family. Had he sent a message to Yaakov immediately upon ascending to power in Egypt, he would have then retaken his place as a member of the family. Even assuming that he would be unable to leave Egypt and return home, he would nevertheless be included once again among Yaakov's sons, and most likely be considered Yaakov's favorite, as he has been previously. Yaakov's sons would never have accepted this arrangement, and would therefore have "fled," or renounced their own membership in the family. Yaakov would then have to choose between Yosef and the other brothers – a decision Yosef did not want to force upon his father.
Yosef therefore devised a plan to place his brothers in a situation where they had to protect Binyamin, Rachel's son, Yaakov's new favorite. As Abarbanel writes, the brothers' teshuva process was then complete; they atoned for their mistreatment of Yosef through their efforts to protect Binyamin and return him safely to their father. Only then was Yosef prepared to reveal his identity, as he was assured that his brothers would now embrace him as a full-fledged member of the family.