While safety is an important concern for most women, they feel unsafe with a nervous guy because he won't be able to protect her 'when push comes to shove' and the self-assured confident man feel safe because he's able to protect her.
Another thing has to do with responsibility and accountability. Women want a man who can take control of them so they don't have to be responsible and accountable anymore. They feel that the self-assured confident man can take care of them when they give over control, but the nervous twitchy guy doesn't have enough control for them to submit to his control.