Supply and demand. Bitcoin used to be a niche, only followed by a few hackers and enthusiasts (low demand). At the same time, by design, mining was easier (high supply). Now Bitcoin is becoming more mainstream, and even the skeptics are starting to buy in, both literally and figuratively (increasing demand), at the same time that mining is becoming harder (decreasing supply). As Bitcoin gets wider acceptance, and as mining gets more difficult, the price will continue to rise, until it reaches a plateau.