The explanation doesn't clarify much
The Legend of Zelda: Breath of the Wild’s Hyrule was packed with hundreds of shrines, guardians and towers, mysterious relics of the past that tied into the game’s core plot. However, nearly all of that technology had completely vanished by the start of the game’s sequel, Tears of the Kingdom, with no in-game explanation. Well, as noticed by Twitter user ZeldaLoreYT, the game’s director Hidemaro Fujibayashi finally revealed what happened to it all in an interview with UK news outlet The Telegraph earlier today. Here’s what he had to say:
Essentially, it just disappeared. Nobody in the game’s universe knows why, and seemingly nobody even cares. It’s a bit of a disappointing answer to such a big mystery, but at least we have some sort of explanation as to why Tears of the Kingdom’s Hyrule lost many of its landmarks.
I wish they had spent just a bit more effort making the game make sense with the rest of the games around it. It's crazy that this question wasn't answered in-game, and frustrating that the answer is "nobody knows".
I'm suspecting they also don't know how this game fits in the timeline.