It's the network affect. More people using it, the price rises, more people buy, the price rises more, and on and on.
The price was actually comparatively low as of Q1 2017
Note the actual comment I'm responding to:
> The more people that are willing to accept Bitcoin in exchange for goods and services, the more valuable Bitcoin is.
Approximately nobody accepts Bitcoin in exchange for goods and services except to buy drugs. Which does count as a use case, but it's a trivial one compared to the speculation; and even drug buyers and sellers are endlessly frustrated by the transaction clogs.
Wallet activity continues to grow
[1]:https://blockchain.info/charts/n-transactions?timespan=all
[2]:https://blockchain.info/charts/n-unique-addresses?timespan=a...