Jay Gatsby is well-loved during his life because of his lavish lifestyle; he becomes the new hot-ticket party and social affiliation, and all the rich people in the area, especially the snobbish East Egg folk. Being seen with and around Gatsby is a social coup and his wealthy associates, such as Mr. Klipspringer and Mr. Wolfshiem, take pains to mention his name and spread the word that they are friends. After Gatsby's death, however, his status falls; he is no longer considered worthy of attention (not because of his death, but because of the stigma surrounding it) and so nobody attends his funeral. When Nick visits Mr. Wolfshiem to try and convince him to come, he receives a response that likely explains many of the absences:
"When a man gets killed I never like to get mixed up in it in any way. I keep out. When I was a young man it was different--if a friend of mine died, no matter how, I stuck with them to the end. You may think that's sentimental but I mean it--to the bitter end."
"Let us learn to show our friendship for a man when he is alive and not after he is dead," he suggested. "After that my own rule is to let everything alone."
(Fitsgerald, The Great Gatsby, guternberg.net.au)
Part of this is Wolfshiem's shady business practices; it is not a good idea to associate oneself with a murder, even if there is no actual connection. The idea gets around and it becomes harder to work with suspicious people. The other implication is that Gatsby was simply a tool on which the idle rich could freeload and boast; a dead man has no such prestige, and so cannot help promote a person's own social status. Once dead, Gatsby serves no purpose at all, and so those around him -- Jordan Baker, Daisy and Tom, all the people at the parties -- simply choose to forget him. His usefulness is over, and so he is forgotten.