How about: "lots of people who bought at the top of the BTC hype are now abandoning the BTC market because they lost faith in their get-rich-quick scheme?"

The meteoric rise and slow but steady deflation in the BTC price you see now has all the properties of a classic, media-fueled bubble. At one point people who were never interested in monetary systems or cryptocurrencies at all, were asking me how they could buy BTC because they heard on the news it went from $100 to $500 in a few weeks. These are not idealists or miners, but just hoped to ride the hype train up and make a few quick bucks without actually having to work for it.

I don't think it's 'difficult to determine' why it's fading now. Maybe if you completely live in a kind of echo-chamber that HN appears to be on topics like this, but looking from the outside it all makes perfect sense.