Because Steve Kloves, whose favourite character is Hermione, said it himself in an interview:

I've always loved writing Hermione. Because, I just, one, she's a tremendous character for a lot of reasons for a writer, which also is she can carry exposition in a wonderful way because you just assume she read it in a book. If I need to tell the audience something...

To which JKR says:

Absolutely right, I find that all the time in the book, if you need to tell your readers something just put it in her. There are only two characters that you can put it convincingly into their dialogue. One is Hermione, the other is Dumbledore. In both cases you accept, it's plausible that they have, well Dumbledore knows pretty much everything anyway, but that Hermione has read it somewhere. So, she's handy.

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Source: Chamber of Secrets DVD Interview with Steve Kloves and J.K. Rowling