12 years ago
Because she was part human. Ripley and the queen were cloned from DNA found mixed together, and they weren`t perfect clones. They accidentally gave the queen a mammal reproductive system along with her old one, just as they gave Ripley acid blood and a different muscular structure (as you can see from her increased strength). As the queen became pregnant with the newborn, she stopped laying eggs, but they didn`t know what was happening to her, because apart from being a Xenomorph, she was a new kind of Xenomorph that was even xeno-er to them.