Why does the party end with the car crash involving the owl-eyed man?

Answer 1

The man in the crash was the one in Gatsby's library, he was drunk and some how got out of the house and drove into another drunk driver. The party was already ending prior to this but officially ended at the time of the crash.

Answer 2

The car crash involving the owl-eyed man symbolizes the collapse of the Jazz Age extravagance and decadence. The owl-eyed man, who symbolizes insight and awareness, is a witness to the emptiness and superficiality of the party culture, leading to the tragic end of the party as a metaphor for the moral decay of society.