Sandy’s parents send her to Florida to keep her from marrying Soda.

Sodapop Curtis is Pony’s older brother.  Pony loves him to death because he is such a kind older brother, but he is also handsome like a "Greek god come to earth" (Ch. 1).  Unlike their other older brother Darry, Soda doesn't yell at Pony.  He loves horses and works at a gas station and hangs out at rodeos.

Soda is head-over-heels in love with Sandy.  Soda is a great guy.  He is heartthrob cute and lovingly sweet.  Since he has a big heart and not many prospects as a greaser, it sort of makes sense that he would fall in love at an early age.

"I think I'm gonna marry Sandy. After she gets out of school and I get a better job and everything. I might wait till you get out of school, though. So I can still help Darry with the bills and stuff." (Ch. 1)

However, sixteen is way too young for most people. When the girl’s parents found out, they were not pleased.  For one thing, Soda was a high school dropout.  He also had no parents.  He probably did not look like prime husband material. 

When Pony asks Soda if he is taking Sandy to a party, he learns that she has been sent to live with her grandparents in Florida.

"Look," Steve said, surprisingly angry, "does he have to draw you a picture? It was either that or get married, and her parents almost hit the roof at the idea of her marryin' a sixteen-year-old kid." (Ch. 7) 

Soda, miserable, responds that he is almost seventeen.  It saddens Pony because he knows that Soda really loved Sandy.  He never bragged about her like he did other girls.  He just cared about her, and losing her would break his heart.

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.

One of the Curtis brothers in S. E. Hinton's novel, The Outsiders, Sodapop was in love with his girlfriend, Sandy. Apparently, Sandy becomes pregnant, and she moves to Florida to live with her grandparents. Soda proposes marriage to her, but Sandy turns him down (or gives him no answer). Soda writes to her during her stay in Florida, but the letters are returned unopened. It is implied that the baby is not Soda's, and that Sandy has been cheating on him. Sandy's parents probably sent her to Florida to either keep her away from the father and/or to keep her pregnancy a secret in Tulsa.

We are not exactly told why Sandy moved to Florida.  However, if you read between the lines of the story, it seems pretty clear that she moves because she is pregnant.

You can see this because of what is said in Chapter 12.  There, Darry says "When Sandy went to Florida ... it wasn't Soda..."  We don't know what "it" is, but I assume that "it" refers to who the baby's father was.  That's why Soda was so devastated.

We can also infer that she was pregnant because Soda offerred to marry her before she left.

Remember that in these days (the book is from 1967) it was really quite shameful to get pregnant without being married.  So it's realistic to think that someone would move if she got pregnant.  And it's realistic to think that the guy would offer to marry her if he thought it was his.