Because of supply and demand, it is simple as that, if a lot of people starts to invest in bitcoin, the price will go up, if they all sell, the price will inmediately go down at that moment, it is exactly what it is happening right now (going down, like hell, lmao)
Indeed, in the simpliest answer it is just because of supply and demand. As the time passing by bitcoin becomes more popular and lately it is being used in any kind of transaction whether oline or offline. Folks are tend to used this currency just because they have been seen the capability of this what we called bitcoin.