Both the Jewish and Roman societies of the time were strongly patrilinear in nature. This meant that family identity and inheritance came from your father only. Under the Roman system, a wife legally remained a part of her father's family. Part of the drama of the book of Ruth stems from the complicated patrilineal system of the Hebrews at the time. Naomi expected Ruth and Orpah to return to their own people after her husband and sons died:
But Naomi replied, “Turn back, my daughters! Why should you go with me? Have I any more sons in my body who might be husbands for you? Turn back, my daughters, for I am too old to be married. Even if I thought there was hope for me, even if I were married tonight and I also bore sons, should you wait for them to grow up? Should you on their account debar yourselves from marriage? Oh no, my daughters! My lot is far more bitter than yours, for the hand of the Lord has struck out against me.”—Ruth 1:11-13 (NJPS)
Outside of rather extraordinary circumstances, one could not expect to leave their fathers. So Mark 10:29, which implies we can leave our fathers, is remarkable. But gaining 100 new fathers would have been unthinkable. The closest modern example I can think of would be if I renounced my US citizenship in exchange for all the rights and responsibilities of 100 other countries. That would seem more of a curse than a blessing.
Matthew, who almost certainly had Mark at hand when he wrote his gospel, cleverly edits the teaching to avoid the question of what one gets in return for giving up everything for Jesus' name. The principle remains, however: if you give up what you have, God will return much more to you. In both passages, the context is the rich, young man that refused to follow Jesus rather than give up everything he had.
God is the new father
When Jesus began his ministry, there is a good chance that Joseph was already dead.1 Since Jesus was the oldest son, under the patrilineal system he would be the head of his family. Shortly after Jesus called his disciples, we have this very strange story:
And his mother and his brothers came, and standing outside they sent to him and called him. And a crowd was sitting around him, and they said to him, “Your mother and your brothers are outside, seeking you.” And he answered them, “Who are my mother and my brothers?” And looking about at those who sat around him, he said, “Here are my mother and my brothers! For whoever does the will of God, he is my brother and sister and mother.”—Mark 3:31-35 (ESV)
His family was basically calling Jesus back to his duty as head of household (it must have been clear he had stopped running the carpentry shop at this point) and Jesus essentially rejects them.2 His definition of his own family is centered on doing God's will.3 Immediately after redefining family, Jesus tells a series of parables about how the kingdom of God will grow. When he explains a parable about a sower casting seeds, he specifically mentions that "the ones who hear the word and accept it [will] bear fruit, thirtyfold and sixtyfold and a hundredfold."
Jesus reassures his disciples that giving up family and property for his sake will not go unrewarded. We will get back all the we lost by joining a new family with a hundred times as many siblings and parental figures than we had before all under the fathership of God Himself.
See Mark 6:3, which mentions Mary, four brothers, some sisters, but not his father.
Thankfully, we know that at least his brother James became a leader in the church. Otherwise, this passage would be altogether painful.
Interestingly, Luke 8:19-21 defines family as "those who hear the word of God and do it" and Matthew 12:46-50 says "whoever does the will of my Father in heaven is my brother and sister and mother." It seems like the gospel writers worked hard to get Jesus' meaning across to their respective audiences so that it would not be misunderstood.