Publishers now have Directs, PlayStation showcases and their online events that they can use to announce their games, though things like TGS can still have a place for people in the business to meet and network.
Publishers also don't love being one news story amongst a thousand others that day.
They like to have a day they can call their own. It took them a while to realize that they had the power to brush off to crowds of thousands and national/international press opportunities (I think there was also a little fear that you don't just do this, turn your press releases into press and hire influencers who don't just speak the company line but who make it look like it's their personal opinion on the product that gave them their smile,) but eventually they figured out how to 100% control their product presentation.
Wow TGS is really dead. The glory days of the early 2000s seem like ancient history.
Timing on TGS (and Gamecom now) was always weird, being at nearly the end of the gaming year.
Early 2000s were big for TGS because they could announce a game in September and release it in November/December and that would work. Famitsu would cover the games, the TV marketing would spin up, and the hits would drive word-of-mouth. That worked real well when game development cycles were 12-18+ months and they could keep it under wraps for that long. Then games started going longer, and they had to have more games announced at TGS that wouldn't be out until the next TGS. That worked okay for big publishers like Square Enix (especially since they could put a MegaTheater together and game all their holiday games get seen while people frothingly showed up for a tease of the next Final Fantasy,) but not many other studios could show up and only talk about a game, having nothing to sell or play. (TGS is also a public event, so part of the experience is letting kids line up for the games and seeing which ones ran long lines; I would imagine demos also hurt that, although I know a few times the longest line at TGS was weirdly for games that were already or about to be out.)
Now, what is TGS's purpose? It's at the end of the year. It's for games mostly being made for a struggling game development business on the international market (Japan still buys a ton of games but those games rarely reach the rest of the world in the same way.) It's hosted on Japan-time, at an hour when much of the rest of the world is sleeping or not paying attention. It's full of titles that are or will be published by international conglomerates who have better exposure opportunities at other times of the year. It's trying to promote games to an audience who are now way more into their phones than their home gaming consoles. (Thankfully, there's the Switch.) It's not a good time to do a show that's not going to be that big for an audience that can watch everything debuting there from the comfort of their homes. TGS has found its level, as a fan event and as a showcase for Japanese titles to keep an eye out for, but as far as being a megaton event for a slate of major games, TGS just doesn't make sense for that these days.
