I think its because there are two basic kinds of main characters in mainstream superhero comics, those the readers want to
be and those they want to
do.
Even the most ostensibly "strong" woman in the comics ends up an exaggerated and sexualized being with little to make her appealing beyond a superficial independence and an adolescent obsession with sex and violence.
Heck, there was a time that even Kitty Pryde was being drawn with D-cups.
In my view, these comics appeal to a group of 30-something men whose view of women was in part shaped by their obession with comics as adolescents, except that they have never grown out of that childish manifestation of desire.
And I say this as a guy who buys comics every Wednesday.
I write about comics some on my blog. Examples:
Comic Books: A Second Adolescence
Excerpt: [T]he ease with which we can collect and absorb the historical narratives of these characters in this age of the internet makes retconning more pervasive because it is too easy for people to reference contradictory events in the Marvel Universe. It gets harder and harder to just pretend something didn't happen or never existed because of this access, so very trite explanations that bind together a variety of things have to be come up with. If my generation had simply outgrown comic books like the generations before us, it would not be as necessary to make these explanations, but we didn't.
Of Man-Drills and Spider-Women
Excerpt: The current crop of mainstream comic book writers are in a way their own audience. They are guys in their 30s/40s who are finally getting to push comics towards what they desired when they were adolescents without crossing the line (too often). And that audience is (for the most part) guys in their 30s - Guys who can afford to buy comics week after week that can total more than $100 a month. These are guys who have never gotten over that idealized vision of women from those teen years. I know because I am a guy in my 30s who is in Forbidden Planet every Wednesday picking up comic books (though I am at less than $25 a month). I know because I am guy that is only slightly ashamed to admit that if I had a girlfriend that could fill out a Spider-Woman or Wonder Woman costume and tie me up, I would be pretty happy in that moment. I know because I found an interview with Brian Michael Bendis (who writes New Avengers) in which the first thing he mentions when asked about why he loves the character of Spider-Woman is "For starters she's way hot, and she's got a great costume, and the best head of hair in comics." The dude is 40.
EDIT: Warning, these blog posts are not always grandma-friendly, and some may verge on NWS, depending on where you work (no full nudity, but ya never know. .. )