How does Ralph treat Piggy? Why do you think he treats him like that?

In order to answer this question, it helps to know when in the story the question is referring to. Ralph's attitude toward and treatment of Piggy changes over the course of the story. When Ralph and Piggy first interact with each other, Ralph is not especially kind. He's outright antagonistic in fact, and he acts aloof and maintains an "I'm better than you" type of attitude. This treatment starts right away. Piggy asks Ralph his name, but Ralph does not reciprocate the question. It's as if Piggy and his name aren't worth Ralph's time because he has already judged Piggy not his equal.

"What's your name?"
"Ralph."
The fat boy waited to be asked his name in turn but this proffer of acquaintance was not made; the fair boy called Ralph smiled vaguely, stood up, and began to make his way once more toward the lagoon.

Ralph is even insulting toward Piggy's family:

"Sucks to your auntie!"

Ralph then rolls around on the ground laughing hysterically when he finds out that people called Piggy "Piggy" back in school.

He dived in the sand at Piggy's feet and lay there laughing.
"Piggy!"

Ralph probably treats Piggy that way because that is how he probably always treats people that he doesn't consider his equal. Ralph is described as a fairly good-looking and capable kid. He's used to getting his way and having people look up to him. His initial treatment of Piggy is his teenage way of letting Piggy know that there is a dominance hierarchy on this island that is based on looks and fitness. Fortunately for Ralph, Piggy is a fairly forgiving and loving individual. As Ralph's ruling power begins to dwindle in the face of Jack's power grab, it is Piggy that stands by Ralph, and Ralph is grateful for that. Consequently, Ralph treats Piggy much more cordially as the book continues.

Ralph is initially rude and dismissive towards Piggy at the beginning of the novel. Ralph is relatively unsympathetic towards Piggy and calls him by his embarrassing nickname in front of the boys. Ralph also splashes water in Piggy's face, does not reciprocate his friendly advances, and does not allow Piggy to come along while they explore the island. One could assume that the reason Ralph is rude to Piggy is because he realizes that Piggy is an outsider. Unlike the other boys, Piggy is physically weak and overweight, has poor vision, and is relatively annoying. Ralph more than likely avoided unpopular, irritating boys like Piggy in school, which is probably why he initially treats him poorly. As the novel progresses, Jack and his hunters gradually descend into savagery and Piggy is one of the few boys Ralph can trust. Piggy and Ralph's relationship grows, and Piggy becomes Ralph's closest ally.