While I am a noted person who loves his elementals, let's not even pretend the various elemental planes were interesting. I think the most milleage was that one time that person in Dragon magazine took over Greyhawk and used its gods to mine the Plane of Salt to sell elsewhere, and I say this as probably like the one person in the world who remembers or likes the Facets (The salt-creatures who can combine together). They're not 'complex', they're square filling, and sometimes dumb square filling (I still have a bone to pick with the Plane of Steam and the multiple bits and bobs where the book goes 'oh no, its not actually the Plane of Steam despite the name, but someone wrote it in a book years ago so despite Plane of Mist being the more appropriate name we're not changing it). The Elemental Planes in general aren't places you really want to visit, but moreso some variety of hostile landscape that blocks you from getting to your particular destination, and, well, an infinite expanse of Progress Blockers isn't all that interesting
Elemental Chaos gives some interesting stuff as by mushing it all together into one, you've got choice. Sure you can cross the Water to try and get to the Tower of Salt, but given that's going to be a constant war and have no subtlty to the approach, can always take advantage of the fact salt is a mineral and find your way in through an alternate path, perhaps a long abandoned tunnel linking to more earth related sides of things
(Also like, let's also just be honest, there's an easy in-canon explanation: In some random Planescape creature book there were these worms one of the Guilds released in the elemental to bind the elemental planes together. Worms won. There's your explanation)