Since Jay had...
The reference at the beginning of the chapter reads:
It was when curiosity about Gatsby was at its highest that the lights in his house failed to go on one Saturday night — and, as obscurely as it had begun, his career as Trimalchio was over.
Since Jay had now started an affair with Daisy, he firstly, did not want undue attention drawn to him and thus to Daisy, and therefore stopped his ostentatious celebrations. Secondly, the parties were only hosted anyway, in an attempt to pique Daisy's interest and draw her closer. Once Jay had succeeded in drawing Daisy in, the parties had become redundant.
The only value of the allusion to Trimalchio is to indicate the over-the-top nature of Jay's parties. They were hugely flamboyant affairs and gave all sorts of weird characters access into his lavish home, whether they had been invited or not.
The fictional character, Trimalchio, whose name means, 'greatest king' featured in a chapter of a Roman work, Satyricon, by the author Petronius. In it, Trimalchio is described as a flamboyant character who had become a freeman and heir to his master's great fortune. He used used his fortune successfully in a variety of entrepreneurial ventures and boosted his wealth.
Trimalchio hosted excessively extravagant dinner parties in which he would brag about his success as a businessman and his great prosperity, to the disgust and envy of his guests. The purpose of his ostentation was therefor only to display his good fortune - there was no other purpose. The whole idea was just pure self-indulgence, unlike jay Gatsby's parties.
Jay hardly ever indulged in the frivolities at his festivities and remained aloof, always hoping that Daisy would, at some or other occasion, turn up. This led to all sorts of rumours and gossip about him, his activities and his past. Trimalchio, in contrast, was the centre of attention at his massive get-togethers. He indulged his and his guests' every whim, getting drunk and expressing his magnificence. The parties were essentially about him and his excesses. He would be incessantly boastful, whilst Jay, on only one occasion, actually bragged about his achievements, in a very private manner when he showed Daisy his huge collection of shirts.
Jay was a private person whose primary focus was Daisy, whereas Trimalchio was an exhibitionist who relished the attention, envy and admiration of many.