Demand is the big thing.
Back when the silk road was around there was millions of dollars of bitcoin going through there and driving the price up.
Silk road was abolished and a lot of the demand for BTC went with it and the price slowly went down since then.
I'm not saying Silk Road was 100% responsible, but it certainly played a big part.
Just my thoughts.
There's no shortage of other dark markets out there. Many of them have volumes in the tens and hundreds of thousands of BTC. Silk Road was definitely the mainstay of the economy for a long time, though. It's far more diverse now.