Because Bitcoin was still an emerging technology and it's future was uncertain, meanwhile there where not much support for it , meaning nobody was willing to accept bitcoin as payment, so buying bitcoin and unable use it made it less attractive for investment, until recent times, now most people do use it in the darknet market maybe it success might be attributed to it use in the darknet market place... people started to see the value of Bitcoin and its usage outside of the darknet market and it came with a lot of incentives for miners as well as for individual with ease of use.. and there became a surge in bitcoin and fast forward today this is how far bitcoin had gone and why people still buying.