It's because most people in the 1600s-1800s are actually noobs from the 1300s-1500s.
They don't even know how to play, yet they are winning because elemental is overpowered and because other players lack the experience to properly counter.
I reached 1700 by playing Order alone before I went and brought premium, not everyone who plays premium empires gets into the top replays by premium alone, and I still can't play Chaos, I nearly always lose when it's involved in my match regardless which side plays it. The empires are not instant win at all.
I have noticed recently that people even those with reasonable grades e.g 1600 and some even higher (Ive seen 1800's do this)
Are blind to the idea of combining units
Now that I've played Elementals, I think I know why. Usually I make very few advanced units and actually only need 1 miner praying once I get a decent bank of mana built up.
It's certainly not that I can't play Elementals, but the advanced units are very singular in what they can do, Charrogs are great tanks but deal next to no damage, and the fire mage deals good damage but only on an opponent that is sleeping or blind or gets tricked. I use both, but I can't rely on either to win a fight, none of Elemental's advanced units are especially good at dealing damage continuously (except maybe the V's clones but that's too expensive for an entire army) like the Air and Fire basic units are.
The basic units make much better armies armies for continual fighting, and give the elemental player much more chice mid fight, if I need to AoE my opponent I can go an make Scorch or the fire mage mid fight, but if I went into that fight with a fire mage only to find my opponent Spearcher totally doge it, I just wasted a Fire and Air elemental.
I have actually managed to lose a match before from making too many combinations and having tonnes of utility and tanky charrogs but no continual damage.
Where as Speartons and Albows make up your continual damage in a fight for Order, for Elementals it's really your Air + Fire.
This is totally ignoring the fools that would try to kill someone with Earth mass in the mid game, but even massing the Water elemental can be very effective - provided you have enough army stuff to make your opponent fall back anyway, the water elementals are very good at denying the opponent running away with live units, and if you have enough of a foothold you can deny the opponent any chance of spell casting with Water spam - but only once you have a competent army anyway.
TLDR: (Counting Charrog as a basic unit :3) Basic elemental units apply the continual pressure on the enemy for mid game and late game, where as Order basic units are more basic in that the advanced units do everything the basic ones do and more. The elemental advanced units are for utility.
Just a 1700 posting though, maybe I'll get wreaked one day by an Elemental advanced unit spammer and change my mind.
In 1200-1600 or even 1200-1700, people will just mass spears because it works, but at 1700+, people try new things which has lead to the development of things like the Magikill Mass, which can actually be very damaging I heard.
Hey, I massed Magikill back when I was 1500 or something. :D Too many times I couldn't finish casting spells before they came back up.