I think It'sbecause of supply and demand...
Bitcoin is a good technology offering so many good an useful features such as anonymity, decentralization, security and high speed. and because of these the demand is constantly increasing.
and while there is only a limited supply of 21 million coins (16 million currently mined) there is 7.5 billion people in the world which are slowly finding out about bitcoin and investing in it hence increasing the demand ...