Let N be the number of great scientists. It would be sensible for the curriculum to include the n greatest scientists, where n << N. Since Marie Curie always gets included in the textbooks, we can conclude either that she ranks as one of the n greatest scientists, even for very small n, or that the reasons for her inclusion go beyond her scientific accomplishments. Considering how few history textbooks mention (for example) Euler, Gauss, Lagrange, Laplace, Lavoisier, Faraday, Dirac, Onsager, or Landau---whose scientific accomplishments all rival or surpass Curie's---it's safe to conclude (as Paul did) that the textbook writers include some non-scientific factors when deciding which scientists to include.

Let's face it: almost no one reading this knows who the hell Onsager and Landau are---but you probably should. (They were both giants of 20th-century physics.) No one disputes that Marie Curie was a great scientist, but that's not enough to account for her ubiquity in the textbooks. And since the most significant diff between Curie and Landau is gender, it's accurate to say that Curie gets included because she was female.