I'm no expert on crypto market.. but IMO it seems, in game theory terms, miners are flooding the market with zcash (at any cost it seems) driving the price down. That is too bad because counter intuitively, if miners held longer it would reduce supply on the market and keep prices higher longer. The Nash equilibrium here is that no miners feels he has anything more to gain by holding Zcash but more to gain by selling as fast as he produces.
All that is very obvious I'm sure . Miners should take a page from DeBeers (the largest diamond producer): There's an over supply of diamonds, but DeBeers keeps prices high by hoarding.