Because that IS the Rede. 8 words, that's it. (Eight words the Wiccan Rede fulfill). The Rede has been incorporated into several nice Wiccan poems, one of which is probably what you're referring to as "the rest of the Rede". Something like:
Bide the Wiccan laws we must
In perfect love and perfect trust.
Eight words the Wiccan Rede fulfill:
An’ ye harm none, do as ye will.
What ye sends forth comes back to thee
Ever mind the Rule of Three.
Follow this with mind and heart,
And Merry ye Meet, and Merry ye Part.
I have the above poem printed in nice lettering on parchment paper and framed on a shelf in my living room.
And there are some even more elaborate, like this (which is the first page in my personal Book of Shadows):
BUT, the Wiccan Rede itself consists of those 8 words: "An ye harm none, do as ye will" or whatever variation that says the same thing. e.g. "An it harm none, do what you will"
For more info on the Wiccan Rede, please read this article:
Yeeah LWW....no clue where in the asker's question or details that Camille pulled "Christian Wicca" from. I scratched my head in confusion for a moment there too when I saw that answer.