Why wasn't there a big party after the rumble?
The emotional state of the characters following the rumble was far from that of excitement and enjoyable. If anything, there was a somber mood encircling the Greasers because of Johnny – Johnny is in the hospital and is in serious condition. The boys know that he might die and this brings the somber mood to what might have been a joyous event for the boys after they won the rumble. Fittingly, after the rumble is over, Ponyboy and Dally go to visit Johnny in the hospital and he ends up dying. At this point in the novel, the Greasers really do not have much to celebrate.
If you remember before the rumble Steve says he's gonna throw a party and everyone's gonna get ripped. Then Darry says "Where you gonna get the dough little man?" and Steve says "I'll think of somethin'" and they all go on.
I also think half the reason they didn't have a party is because when Ponyboy came home they found out Johnny died and then Dallas dies just a few minutes later.