Because Banks in Britain and the United States have banned the purchase of cryptocurrency by credit card. This is bad news makes bitcoin and cryptocurrency market are going down. 
It's nothing to do with that. Most people don't buy bitcoin with credit cards. They wire cash to teh exchanges.
No, what we're seeing is plain profit taking. You can see the same on the stock markets. The US economy is overheating and interest rates are going to rise. That makes cash savings more attractive, and less risky.