Because the intent behind the usage can and does vary depending on who is saying or using it. Historically, men have used the term to degrade or insult women, so a lot of the time women can be more sensitive when it's coming from a man as opposed to using it among other women as (like many other minority/underprivileged groups, it has been reclaimed and repurposed as an endearment or empowerment statement in some contexts). Bottom line, with any word that is used both as an insult and in a benign manner, intent is everything. That's why it's just better to allow underprivileged groups with reclaimed slurs just have those words. As a straight woman, I would never feel comfortable using homophobic slurs even though I've seen many gays refer to themselves and each other as such -- it's not my word to reclaim and it's not my place to question the usage. It doesn't kill me not being free to use certain words without social repercussions and knowing that if I say a certain word, it might be received differently if someone else uses the same word. Who you are and your background does play a role in the context and intent (or perceived context/intent) of what you say.

That's just life.