Customary, so that passers by would know why they had been so executed.
"It was the custom of the Romans to use gypsum letters written on a rough board affixed to a cross to proclaim the reason why a person was being executed, although three languages were not always used." Source: http://www.christiananswers.net/q-aig/aig-t001.html
The charge that Pilate chose is also called a "title" by John in John 19:19-20:
"And Pilate also wrote a title, and put [it] on the cross, and it was written, `Jesus the Nazarene, the king of the Jews;' 20 this title, therefore, read many of the Jews, because the place was nigh to the city where Jesus was crucified, and it was having been written in Hebrew, in Greek, in Roman." (YLT)
The title or charge was written in Latin (legal), Greek (the international language of the day), and in Hebrew (the religious language of the Judeans).