Its because bitcoin is anonymous .So it makes really easy for a person to hide his black money.Government wont be able to find anyone's money.So person using bitcoin will escape from taxes. Again for transfering money from one part of the world to another , all systems including banks and paypal require a large amount of processing fee ,but bitcoins requires only less amount that too no one can trace your transfer. So definitely people prefer bitcoins over fiat for all these things.