Probably already been said but Zero Dawn was a new property with a quasi-original idea (though not an original gameplay loop) that actually told a decent story with a pretty good twist toward the end.
Forbidden West is now a “known quantity”. The hype of it being a new universe no longer exists and the game itself suffers from quite a bit of sequel-itis. They try too hard to tell a story equal to what they wrote with ZD but it just ends up jumping the shark (to use an oft overused phrase) repeatedly. The gameplay doesn’t stand out over its predecessor either, especially since they doubled down on the amount of open world fluff compared to ZD.
The drop between the two really doesn’t surprise me, especially now that the game’s been out a few years on PlayStation platforms with in depth reviews.
Edit:
My memory is crystal clear, this was just a terrible sequel and a dull experience, with no memorable moments and it felt like a waste of my time.
The first game, the story really grew organically as you went along. And built to a crescendo when you finally learn what Zero Dawn actually was, who Aloy actually is, and what Hades actually is.
The Forbidden West writers felt like they needed to top that and ended up throwing a bunch of over the top nonsense at us.
REGALLA! ZENITHS! QUEN! NEMESIS!
The end result was a game that wasn’t nearly as memorable as the first and has driven the franchise into a weird direction. I would have enjoyed the game so much more if it had just been about Hephaestus being rogue and not all the ridiculous attempts at plot twists