The reason would be the same as that of people invest in share market. Everyone want to make money real quick and shares provide it the same way. Either you win or loose, two things may happen. Same is the case with Bitcoins. In the past few years, Bitcoin has shown a tremendous change in its value which is grabbing the attention of people. Found this helpful post on google which i think answers this question really well. http://theconversation.com/four-factors-driving-the-price-of-bitcoin-87244