Why’d she do it? That’s what we always ask when a powerful White House aide like Karen Hughes suddenly resigns.

Longtime Washington observers are as cynical as the Austrian statesman Metternich who, when informed that the French ambassador had died, said, “I wonder what he meant by that.” Nothing, you see, would be done without calculation.

So, why is Karen Hughes leaving the White House after just 18 months? Did she lose a power struggle with Karl Rove? Is she fleeing an indictment? Did President Bush get tired of being told what to say?

After all, Karen Hughes is the most powerful woman ever to serve in the White House (with the exception of some First Ladies). Indeed, she may be the most powerful woman in the history of American politics.

Hughes was one of three top aides to George W. Bush in the Texas governor’s mansion. Along with Karl Rove, she has shaped Bush’s agenda, political strategy, and communications. She wrote his autobiography. During the campaign she hired two experienced Washington press secretaries, and fired one. Observers say that when Bush speaks in public, her lips move along with his. She is the most powerful shaper of the words and message of a president of the United States.

So why has this powerful woman had received so little media attention? Hughes has not been the subject of magazine covers, Washington Post profiles, and the like. Women who rose almost as far — like Susan Estrich in the Dukakis campaign, Dee Dee Myers in the Clinton White House, and Donna Brazile in the Gore campaign — got much more “first woman” attention. Why?

The Hughes appointment doesn’t fit the story line. Conservative Texas Republicans don’t make breakthroughs for women and minorities. So the fact that George W. Bush has two African Americans running his foreign policy, and a woman as one of his two most powerful aides, produced no flurry of “breakthrough” stories. Successful women and blacks are expected to be outspoken liberals. When they’re Texas conservatives or moderate Republicans with a military background, the identity groups don’t celebrate them.

There may also be something deeper going on. There’s a tendency to celebrate appointments that appear motivated primarily by the race or gender of the appointee. It’s as if the appointee’s actual lack of qualifications becomes his claim to celebration. When Bill Clinton named three women in a row as his attorney general, it became obvious that he wasn’t offering Janet Reno as the best-qualified lawyer in the country but simply as the best available woman lawyer. But no one thinks Karen Hughes got her job as a political gesture; she got it because George W. Bush trusts her judgment.

Now Hughes is getting the front-page attention she has always deserved because her departure is recognized as big news for White House watchers. So, again, why’d she do it? Is it that she wants to spend more time with her family? We don’t believe that line when it comes from men who leave powerful positions.

But maybe the point is that men and women are different. Women are much more likely to give up power and status in the interests of their family. And that’s a fact that some people can’t accept.

A few years ago, in a speech to a politically correct, feminist board of directors, I said that capitalism — with its emphasis on individualism, property rights, and market competition — tends to liberate women. They gasped. Why, then, they asked, are there so few women running Fortune 500 companies? I mentioned a couple of reasons. First, most CEO’s are in their 50s, and 30 years ago there were few women in business school; as women move up through the ranks of corporations, we’ll see more women CEO’s. But we’ll never see 250 women among the Fortune 500 CEO’s, I said, “because men have testosterone and women have babies.” At that they began fanning themselves feverishly. A former governor almost turned her chair over backwards, like John Belushi on “Saturday Night Live.”

Maybe I shouldn’t have been so blunt. And of course no one should think that testosterone is better than babies; men are different from women, not better. But I think the resignation of Karen Hughes confirms my point: We can’t imagine powerful White House aides walking away from their positions except to take high-paid lobbying positions or to run for office. But that’s because powerful White House aides have always been men. Karen Hughes has broken through the glass ceiling and found that the top wasn’t all it’s cracked up to be.

Why’d she do it? For her family. Seriously.