Proof-of-stake is coming to Ethereum later this year, so there isn't much point investing in a few more months of mining. Casper is the real reason everyone is selling their mining hardware.
The drop in price helps potential stakeholders too.
But the real concern, and IMHO, the reason for the drop, may lie in the next big hurdle, which is creating applications based on blockchain contracts. It is a very different way of thinking about application development, and as such, may need more time for consideration.
More like hypertext and the world-wide web was in 1993 than it was in 1996 and '97.
Casper is expected sometime next spring.