That was my recent impression. I can see why. Carrie doesn't just have a voice, she has a stage presence, and of course she's very glamorous. I happened to watch the entire season the year she was on, and you could just watch her growing into the role as a star.
Interesting that though probably more men than women have won at this point, the female winners (and even some non-winners) have by and large been more successful. Though Fantasia Barrino has faltered lately (largely due to her personal problems, I think--she seems to be going through an Oksana Baiul-type struggle, though like Oksana, she's got talent to burn), she did have a role in a Broadway show, not to mention her own movie and two albums. Carrie and Kelly of course have sold the greatest number of records. Jordin Sparks, a charming singer, has been on Broadway, sung the National Anthem at several prominent sports events, and appeared in magazines. And now runner-up Katharine McPhee is a featured actress in Smash, while Taylor Hicks, the winner of her year, seems to have vanished. Then there's Jennifer Hudson, probably the most successful seventh-place finisher in any competition in history. I have a theory about this. I think that currently, female pop artists tend to succeed as individual singers, while male singers tend to have more success as part of a group or a band. (Interestingly, one of the most profitable male Idol contestants ever is Chris Daughtry, who is the lead singer of his band.) Clay Aiken is the one male Idol contestant I can think of who has made it really big, though certainly several of the others have achieved moderate success.
But remember, my specialty is classical music, and in addition I haven't watched all the seasons of Idol. My theory could be full of holes. Please feel free to contradict me!