Some possible reasons I read:
Merchant adoption through payment processors
While merchant adoption is growing, it is currently mostly through payment processors, hardly any merchants keep a significant portion of their bitcoin earnings. With the broader selection of things to spend bitcoins on, more people are shopping with bitcoins, yet, not necessarily replenish their holdings. This creates a downward pressure on the markets.
Retailers first, suppliers lagging behind
And the retailers are right to immediately turn their bitcoin earnings into cash: With the current price downtrend, otherwise they would have lost significant value, more than they could have afforded on their margins. The problem is: Since retailers don't have Bitcoin holdings, I don't think there has been much request for suppliers to accept Bitcoin. – But without suppliers taking Bitcoin, there is no reason for retailers to keep them.
Stocks are doing very well
I've seen it suggested, that some early adopters with large amounts of bitcoins might be diversifying their holdings into other assets. The stock market is doing pretty well, and there were some interesting Initial Public Offerings recently.
Price is set by exchanges, yet, the big amounts are traded on Over-The-Counter markets
The Bitcoin exchanges are too small a market for the big payment processors to offload their earnings. So, instead, they sell huge amounts directly to interested parties. It has been suggested, that some of those interested parties might be using their freshly acquired bitcoins to push down the price on exchanges, in order to buy more bitcoins cheaply.
We are still in the correction after the Mt.Gox hype and crash
Some think that we are still in the correction phase after the Mt.Gox hype, seeking the fair price after it was massively overvalued.
Regulatory stance still unclear in many countries
Some suggest that the Bitlicense proposal in NY could be a precedent for other US states to follow suit. Quite a few followers of Bitcoin seem to think that it would be very inhibiting to the adoption of Bitcoin in the US if that were to happen.
A lot of people invested a lot of money into mining equipment that is now not earning what they expected, or which they didn't even get. There might be some financial pressure on them to market more of their Bitcoin holdings than they wished in order to pay for their upkeep.
Weak Hands and Impatient Enthusiasts
The falling price is shaking out weak hands. People that bought too high and want to cut their losses. Also, some are gambling to buy back in lower. Some (younger?) enthusiasts might be getting disillusioned, because Bitcoin isn't changing the world fast enough.
Volatility is down, damping speculators enthusiasm
Volatility had been decreasing for most of the year, making it less attractive for speculators to invest. Apparently, this has changed this week with new record trading volumes on the Chinese exchanges according to Daniel Mark Harrison on Coindesk.
Just some ideas, most is gossip and AFAIK, I'm sure there are plenty more.