The planned rumble at the end of The Outsiders gave both greasers and Socs an opportunity to seek vengeance on their friends behalf; the Socs wanted to avenge Bob's death, and the greasers wanted to settle the score for Johnny.
The Socs and greasers choose to have the rumble in a vacant lot in greaser territory, because the lot is a less conspicuous place where they hoped the cops would not break up the fight. Moreover, the lot is convenient for the greasers in terms of location because it is in their neighborhood. Since the Socs all have their own cars, they can easily drive over to the other part of town, but most of the greasers have to walk to the rumble.
They don't like each other and the Greasers want to get justice once and for all. On the other hand, the Socs want to win because then they will get to jump Greasers some more, and have all the freedom they like.
The reason why they are fighting in the parking lot because Johnny killed Bob one of the Socs.
firstly, they dont fight in a vacant lot. Johnny and Ponyboy were just walking because Ponyboy just got slaaped by Darry. On their way they saw some socs.
so here is the story.
Johnny Ponyboy and Twobit were hanging out with these two girl socs in the drive by theatre, the girls were Cherry and Marcia. After the movie, they were walking Cherry and Marcia home but on their way they saw some socs, which were the girls boy friends. The Greasers and Socs were about to fight but then Cherry stopped them. So, then the boys start to go home, to Johnny's house. The parents dont care about Johnny, so when Twobit leaves, Johnny and Ponyboy leave to a parking lot. They fall asleep there and Ponyboy wakes up at 2.00 am and goes home. Darry slaps Ponyboy for coming home late so Ponyboy runs away. Ponyboy goes back to JOhnny and they walk to the park. They see some socs and those were the socs of the socs girls boyfriends. The Socs then start a fight because the Greasers were with THEIR "girls".
the bolded was the most important and i answered your question. But the rest on the other side is just info you need to know what i was talking about. :) hope i really helped