Good question - why don't women negotiate as much as men, in general?
That's covered quite a bit in the thread about "should Justine have written this?" Women are good at negotiating on behalf of other people, but not so much for themselves (as a rule; obviously there are exceptions.) This has a lot to do with socialisation, and good girls shouldn't be pushy or demanding. But also, when women do negotiate, it can work against them; they're not seen as asking for their due (as men will be), but they are viewed negatively for it, and are less likely to get a payrise or bonus as a result.
I think there's also lack of education - no one ever told me I would have to push for payrises and bonuses, and early on in my career, I assumed I didn't have to, because I was told it's all performance-related, so I assumed as long as I was performing well, then I should get payrises and bonuses based on that. And I did get them, but I wasn't getting them at the same levels as my male colleagues - when a pay audit was run on the department I was working in then, I got a 26% rise to level me up. Mind you, when I had questioned my pay a year or so earlier, as a colleague had let slip what he was earning, and I was asking, "if my performance isn't as good as his, why has no one told me?" The response I had was, "It's a sackable offence to discuss pay." Had I had a mentor to explain these things along the way, then I would have dealt with it differently, but it's really only been in the last 5 years or so (in my 30s) that it's been made clear to me. I suspect there are lots and lots more women in that position. (It's got bog all to do with performance in any case, else I'd be on masses more than some of my colleagues. It has a lot to do with who you know, who knows about the work you do, and telling people about it - no point producing the most fantastic work ever if no one's aware of it, when it comes to pay. And satisfaction in a job well done isn't enough to pay the bills.)
Men don't always bother negotiating much either, but if you've got a woman and a man working in the same role, and neither negotiates, there's a chance the man will still do better financially - there's a whole load of unconscious biases around things like men showing managerial talent and assertiveness, but women being bossy, men being promoted for their potential, whereas women are promoted for what they've achieved. That's part of the thing mentioned above, women being given harder goals than men.
I know someone who spent a couple of years fighting for their pay to be made equal - they got it in the end, but said it was such hard work, they wouldn't bother in future. (I'm more stubborn than that.)