Many, many years ago "わ" was written as "は".
The reason? Because it's was actually pronounced as "は".
Now: 川 かわ、回る まわる、柔らか やわらか
Old times: 川（かは）、回る（まはる）、柔らか（やはらか）
However, with time, around 10~11 century the pronunciation gradually changed and became "わ". Which means that there was a difference between the character (letter) and the way it's pronounced. So, it was confusing. In order to avoid this confusion, after the world war II, it was decided to change the rules: all words with sound わ that were written as は)、 will be written as わ. Only particles will be written as "は".
Something similar was decided about を and へ as well.