Because 90's Rei didn't go through the same experiences that Manga Rei did.
That's a "chicken and the egg" answer, I know, but there is no evidence that 90's anime Rei ever fell in love with her father's secretary, or even that her father even existed. Without having him as a politician who has a direct, if largely unseen, influence on Rei's life, Rei is a completely different character. She doesn't have as many expectations to live up to so her personality is completely different.