There are many factors affecting BTC and most of the cryptocurrency to drop that low. One of my theory and opinion about this it is because of the price increase of the BTC last December 2017. Why? First because of the jump of $20k price of BTC more people trade their coin and so much of mining company has opened and started to get BTC. Because of this mining companies the supply of the BTC was increase filling the amount of the demand of the people wants to get some. So the price suddenly drops as the people wanting to have BTC was doubled in mining.
Well i think that's already an accepted fact. I don't think bitcoin was meant to pump that high last year esecially at the pace it did it. Many consider this as still part of the correction though i think several factors also add to why we've been mostly red. Doubts and fear for new investors? Manipulation? Too many fud about regulations?