Of course, the first reason is the acceptance by Japan in spring 2017. However, this is not the only reason. BTC really develops and solves it's problems (lightning and speed of transactions as example) and this is why inestors believe in this coin.
The fact is that not everyone believes in these new projects, lightning, which can increase the speed of transactions, because it seems that few people already believe in Bitcoin and it will be very difficult to develop these strategies. You can see for yourself that many users lost their money during the next and very rapid drop in the price of Bitcoin. After 20,000 dollars, a comma is almost twice as much.