Just like all things, currencies are subject to a simple rule, which is supply and demand. As demand increases and supply decreases, the price increases, and vice versa. For some currencies, such as Bitcoin, there remains a fixed supply that increases on a regular basis, while demand grows rapidly, so the price rises, and vice versa for some currencies with low market capacity, where there is a large supply. Very low and decreasing demand, which causes the price to drop sharply.