Much of bitcoin price is driven by speculation and non quantifiable factors. At the end of the day it is a software/code and you deem some value out of it. It does not generate revenue of its own and neither pay dividends. It is not related to any country's GDP.
Perhaps use an auto-correlation model, with factors for short term change in the price. may be think on the lines of gold.
I agree that a large portion of the price is speculative. However, I consider Bitcoin's price to be more practical to forecast than ForEx currencies, particularly when you're looking to forecast in the short term. My main reason is the supply and many other factors are known and available. Because of this, all one really needs is what's going to drive demand or how to effectively measure that - quantitatively.
What other factors apart from supply are quantitatively known?
I wouldn't say that its easier than Forex. FX long term forecasts has multitudes of macro economic data going in. For the short term, the price history depth makes autoregressive models possible. Neither of these factors are available in BTC.