neo9436 is dead on, too much supply.
Which can also be phrased: No incentive for consumers to buy and use Bitcoin.
The real problem is this. The 'real problem' isn't the 3600 coins dumped by miners.
We are 6 years into this, and consumers still don't give a flying f** about Bitcoin, and have no incentive to buy it.
That is the real problem. Lets focus on solutions for that instead of whining about the miners selling coins.
-B-
You're absolutely right, nothing can be done about supply so demand needs to be addressed.
The price dropping will eventually lead to more demand, but then we will be right back where we are now without new solutions.