Due to DEMAND and SUPPLY. First and foremost, let it be known that prices goes UP when there is more demand than supply, while prices goes DOWN when there is more supply than demand.

Back in 2009, BTC has no value, because there is no demand for it. It was reported that someone even tried to sell his 10,000 BTC for $50, but was not sold. Again because there was no demand for it and there's so much supply back then.

Today, lots and lots of people wants to get hold of BTC. There's so much demand while having limited supply that it's price rose from nothing to all time high of $17,900.

When someone buy or sell BTC, number of available supply changes. That's why prices vary from time to time.