Probably for the same reason that lesbians are not as big of a “hot topic” nor do they generate as much feelings of disgust toward them.
Men tend to be threatened by gay males (and by extension, feminine boys) because they think/believe/fear that they are going to get “hit on” or “raped” or “recruited” or “outed” by them.
On the other hand, men love the idea of lesbian sex. And women and girls simply don’t tend to think about tomboys and lesbians in the same (sexual) way that males think about gay guys and feminine boys.
Most women are happy to have some female friends that act typically girly and like to wear make up and high heels, but they’re also pretty thrilled to have some female friends that can change their own oil, climb a tree and hook up an entertainment center. For most women (I’m going by my own experience here folks so don’t get all upset if you don’t feel the same way) it doesn’t even occur to us that a tomboy or a lesbian would be out to “hit on us” or “rape” us or “recruit us.” We’re just happy to have good friends, whichever kind of package they come in.
And for men, the loss of masculinity is seen as a horrible, dreadful thing, whereas with women, gaining some of traits that are traditionally seen as male traits such as physical strength, not being afraid of bugs, and knowing how to construct things as seen as a positive.
Also the nasty word “faggot” has a much uglier connotation than the word “lezzy”. I’ve told this story before, but within my close knit group of friends, my best friend and I have often been called “lezzies” before and it just makes us laugh. It really makes our 2 lesbian friends laugh, because no one suspects that they are actually gay. Plus playing ouselves off as “lezzies” tends to keep the creepy dudes at bay, when we all go out for a girls night out.