If I had to guess as to why the PC release feels so neglected, it would be based on it's Exclusivity to the Microsoft store period.
Majority of PC players when it comes to Mmorpg's, or just games in general prefer Launchers, such as Steam, and when other launchers/places to find games for the PC come out, there's a huge outcry, as to people only wanted just "one" place to be able to download, and play games.
It took forever for PC players to accept Origin as a place to download games, which then in turn took forever for PC players to accept Ubisoft's launcher, which then spawned every single type of launcher out there, the recent, and biggest outrage being Epic Games launcher.
However, when it came to Mmorpg's, Launchers were nothing BUT a common place to download your games.
That being said, Microsoft Store is probably one of (Of not THE) least place where PC players are likely to get games from. Not only that, Microsoft doesn't really go out of their way to promote their Store, unless they have exclusive known titles, such as State of Decay. I suppose it's becoming a more popular place to get games now though, due to the "Game Pass" being around.
If I also had to take another shot in the dark, which actually isn't too big of a shot in the dark to be honest, the Hype around PSO 2 has been dead for a LONG time. The game had been planned, and talked about a western release for ages before Microsoft decided to pick it up, the game is already 8 years old, and majority of people who wanted to play from the Western Audience already do so on the Japanese version of the game using a translator (and PSO 2 Tweaker), and there's simply no reason to come over to the West to play a game with far less content, and starting over new, compared to a character, or account you might have already spent thousands, if not tens of thousands of hours on.
With the hype being as dry as it is, It's a safe assumption to say, PC players interests lie elsewhere, where as with consoles there's only "so many" games they have access to. I don't think I'd be wrong in saying the console is going to heavily outweigh the playerbase with PSO 2, especially since it's a Microsoft Store Exclusive, which as mentioned earlier, not many PC players are heading to for games to begin with.
While I myself have played PSO 2 (Japanese version), and love the franchise, I already know how quickly content can get old. And after reading the forums for a little bit, and seeing the console version of PSO 2 dying down, it's probably safe to say, much like the western audience on the japanese version of PSO 2, it's going to a very small community, and only a handful of people are actually keeping up to date with this game, and it's news. It just wouldn't make sense to heavily try to over-sell, or advertise a game when you already have some idea of how "big", it's going to be.
This of course it my opinion. However, I've been around since the good old Ultima Online days, and play games regularly (not as much when I was younger), so I think it's safe to say, at least in my opinion, how games are generally advertised, and how well they're going to do, especially since I've kept up to date with PSO2, coming from the Japanese version myself.
I still do plan on playing the heck out of PSO 2 upon the western release tomorrow, I just don't think the playerbase is as big as what everyone on these forums wants, or hopes it to be. It's going to be very small. It would be a different story, if they merged servers with the Japanese Servers. Which who knows, they might end up doing, very unlikely, but we've just entered the Era of "Cross-Play/Platform", so anything is possible.
Long story short- No one is going to Advertise a game, with results they already have a low expectation for, as sad as that may sound.