Why is Primal Earth the most expensive one?

Primal Earth has consistently been the most expensive of the elemental primals on both the alliance and horde AH since I've started checking regularly. I'm curious what the reasoning for this could be.

It's not any harder to farm than the other primals, if it's anything it's a little easier because of the mining source.

Looking at the list of things it is used for, nothing stands out as something that would create a demand for it any more than the other primals. It isn't used for something that is consumed or disenchanted for profit. The item that most stands out would perhaps be Elemental Blasting Powder, but would this really be needed enough to make Primal Earth valuable?

I think that it has to do with elemental blasting powder being a reagent for making flares which are used for raiding and are a consumable.

Plus you'll need them for leveling up engineering and mats for that are always more expensive as people try to power level it. So those two things plus it not being commonly farmed means the price stays higher.