Both Lisa and Marge are also the voice of reason among the craziness. To me, The Simpsons often feels like a "
straight man act," and you can't do the act with the funny guy alone; you need the straight man to relate with the audience just how absurd the situation is.
I also really just genuinely enjoy it whenever Lisa gets intellectual about a subject, even when I don't fully comprehend what she's saying. I enjoy prose a lot.
I also think that I could sympathize with a lot of Lisa's woes. Aside from being a female, she's a vocal female with assertive ideas about how to make the world better. For an eight year old, it's easy for her to get ignored or reprimanded even. In Family Guy, she would be Meg easily if Homer was actually abusive to his family. I also get the feeling that Lisa feels out of place among her peers specifically because of her intellect. Bart couldn't care less because he's a rebel, but Lisa cares too much, me thinks.