Honestly these are probably reasons:
A) People are realizing more and more the 'not-worth-it-ness' of the iLvl 70 crafted gear like Vanya and Rose Gold. It's useful, but certainly not best in slot. Cost in tomestone materials has plummeted, and the ability to HQ those items is easily done by hundreds of competing crafters on a server. Since these items are the only items really *worth* putting battle materia on, as demand for these items decreases, demand for battle materia to put in them decreases.
B) People were buying lots of crafting materia for a long time to gear up their crafters in crafting AF hq gear, fully melded and all. Except by now, a lot of people have realized how little money there is to be 'made' by putting all that materia on that gear. Because no crafted items are really viable or necessary for top tier battle content, people are realizing that crafting as a market has been catering to *itself*, and that the profitability of that pyramid scheme has been collapsing for the last month.
C) The huge exorbitant gil cost of housing has prompted people to seal their purses tighter than Zeus' sphincter to save every last gil for their house, whereas everyone with materia stockpiled is trying to unload it now before the market crashes.
Those are my thoughts anyway.