It takes the New York Times 5 pages to answer the question Why Do So Few Women Reach the Top of Big Law Firms?, but I could do it in 5 words: because big law firms suck.
Is it really any shock that anyone decides to leave a job that makes them work 60-100 hours a week whilst punching the clock like a waitresses (worse, really, because waitresses don't have to punch out when they go pee)? Let me tell you a little anecdote: when I got sworn into the DC Bar, the judge who performed the ceremony gave us 3 pieces of advice: 1. Don't commingle your client's funds with your own; 2. If you find that you are very depressed or abusing drugs or alcohol, you have an ethical obligation to take a break from practicing until you fix the problem; and 3. If you don't spend any time with your family, your children will hate you. Welcome to the practice of law!
The real shocker is that anyone decides to stay in big law firm jobs. I would suggest that the reason more men than women do so is because more men than women feel pressure from society, their families, their own overachieving selves, whatever, to climb whatever Accomplishment Mountain they find in front of them, despite the fact that they'd rather work less so they can spend time with their children, or just so they could leave work while it's still light out.
With that, I think I'd best stop spending time with the computer on Sunday, and go eat brunch.