Because those that founded it, ehum, say that they cannot actually afford to publish it any longer.  Our Bodies, Ourselves was first published in the 1970 ‘s — when the topic of women’s bodies and sexuality, were uh, rather taboo and almost unheard of.   Instead of further updates to the iconic book,  the group, will reportedly transition to volunteer organizations and championing modern women’s rights.
The last update to the book was done in 2011.