Thousands of investors hold Bitcoin on troubled days because they are familiar with the volatile nature of Bitcoin. They know that if there is a huge drop in the price of Bitcoin then surely  there will be a huge boost also.These investors does not commit a mistake of becoming a panic seller and for them these days are the opportunity to buy some more bitcoins for future.  They know that the price will increase again and it will increase more than the estimated value which will provide huge profits to them.For example after that China incident,there was a huge drop in the price of Bitcoin .The investors who sold there BTC's are in a total loss today and those who held there BTC's are enjoying there profits as the price has again reached to 4300$.