Because there are tons of people who care about Bitcoin, how good it is and how potiental it is, so if people are enjoying it they will looking to buy Bitcoin and the price will be pushed to the moon because the quanlity of Bitcoin is limited.
While Bitcoin is expensive it continues to attract more investors and people want to invest because the profit is very high.
Looking at the price graph chart from 2015, Bitcoin never stopped increasing.
First reason is the increasing demand and decreasing supply day by day , so this is usual thing in any field that results into increasing price .
Second reason is the bitcoin fork , here already three fork happened and three coins in equal amount of the bitcoin we got , So people are waiting for new incoming fork and want to make free money , and this is also truth that new 2-3 fork will happen in the next year of Frist two months ( According to news that we seen in many social media ) .That why the people are investing money in the bitcoin in bulk amount to make profit with two side , one by increasing price second by fork free coin.
Seriously, I already see how those who did not believe in the rally before the new year began to tear their hair. This is typical and therefore I do not understand why people do not believe in it. So many times this has already been on other sites and currencies ... I think this pace we will easily see 15k by the end of the year - because human greed is limitless! 
