I'm fairly certain if they decided to do traditional melodic music, they wouldn't have one single theme for the entirety of the overworld, it'd be region based
The music in the field theme is
melodic, in the sense it contains melodies.
Also, consider, BotW has a vastly larger overworld than in previous Zeldas.
The overwhelming majority of your playing time in this 50-189 hours it takes to complete the game is spent in the overworld.
In previous games, this wasn't the case.
Thus, this creates a situation, where, as a composer, you want to make music that sounds good and captures listeners' interests, but not have it be so repetitive, because overworld themes in Zelda games are around 2 minutes in length.
If you're spending several hours in the overworld, 2 minutes of constantly repeating music will be much more grating to the listener.
Sure, maybe we can try something like Hyrule Castle's theme, but let's also remember that the expectation of players now is fully orchestrated music played by real musicians. Paying these musicians, on top of recording them in a full orchestra, on top of recording much longer music in different variations for thematically appropriate reasons is going to be exorbitantly expensive.
Not to mention coding to make sure the music ties in with the intended vision.
This is why a lot of video game music is pretty short compared to say, jazz standards or classical music.
But, if instead of doing something with traditional functional harmony, if we go more with ambience and non-functional harmony, we can keep a listener's interest much better, because the music in this case isn't serving the same purpose that functional harmony serves.
Ambient and non-functional harmony tends to evoke a mood and keep you there.
I explained the purpose of ambient music in one of my blogs before.
So it was a smart choice to go for an ambient piece. Furthermore, think of the context of the story. Strong, bombastic themes are indicative of heroism, might, majesty, and if tied to a kingdom, this conveys the power and might of the kingdom.
But in BotW, Hyrule Kingdom is not
strong or powerful, at all.
So how would one convey its original might and majesty but also show that it's decayed, a shadow of its former glory?
But because it wasn't what people expected, people just thought it was random plinkings on a piano, even though each of the notes are meticulously placed and deliberately composed that way.