The text also states that Sandy's parents did not want their daughter to become a teenage bride. This is why they sent her to Florida to live with her grandmother.
We also learn that Soda did ask for Sandy's hand in marriage. However, Sandy's parents could not get past the fact that their daughter would be marrying a sixteen-year-old boy. In the story, it is Steve who divulges this bit of information to Ponyboy.
To his grief, Soda discovered that Sandy was not carrying his baby. Yet, he offered to marry Sandy because of his great love for her. For her part, Sandy turned Soda's offer down, and it is heavily implied that she did so at her parents' behest.
It can be inferred that pressure from Sandy's parents is the likely reason Sandy also ignored Soda's letters to her. Darry later lets Ponyboy know that Soda cried every night the week of Ponyboy's absence. Soda was devastated because he believed that he had lost both Sandy and Ponyboy. Later, Soda reunites with Ponyboy in the hospital, and Ponyboy comes to appreciate his brother's personal challenges.