Because there are many exchanges and they do not stop their work. Trade occurs around the clock, so the market does not sleep
If you are there in the world of the crypto currencies and that you are observing the things which are happening in this world, then you will get to know that there are so many traders and investors in the market who are continuously making a huge investment and that they are dealing with the things related to that of the crypto currencies in a way which are making this market alive.