Yea it really depends on the supply and demand. Bitcoin price does vary on exchanges coz every exchanges has their own volume, you might notice that bitcoin price in some country is not the same as your exchange price, and there you'll realize that the huge factor of the price is the demand.
The demand is coming from different direction but most are being concentrated to the whales.

Now you might wonder why there is a demand for a digital coin like bitcoin, that's for another topic to discuss.