why does fitzgerald describe the party in the past tense?
If you look at the beginning of the book, Nick is telling the story as a flashback of sorts, the entire thing is in the past tense. So the story of the party is also in the past tense.
Much of the tone of the book is corrective or at least has the feeling of pointing out things that were wrong or things that had at the very least gone wrong, so it makes sense to tell the story in the past tense to amplify the sense of a correction or lesson being taught.
This may very well be due to the act that many incidents in the story are very similar to incidents in Fitzgerald's own life and it would be more comfortable to re-tell those in the past tense.