Short answer: Because nobody has contributed any, or told me we're lacking them. I'll see if I can find some.
By the way, we get questions like this often and there seems to be a misunderstanding behind them. It seems like people think that we pored over all of the world's literature, sifted through every word, and tried to select the most important or quotable phrases. Although this is an essentially impossible task, it's the basic technique that books like Bartlett's Familiar Quotations
have always used.
We're different. This site started out as my collection of quotations that I'd run into in books and magazines. I've continued over the years to add quotes to my collection in the same way - things that I like, from books I read, Web sites, and so on. Now we have Laura's collection, and several other collections assembled by other people - all with the same basic concept, "these are some quotes I like".
This leads to a collection with some obvious gaps. Few quotations from Tolkien or Reagan, but hundreds from Wilde and Shaw and Douglas Adams.
I've started trying to fill these gaps recently in the Classic Collection, which includes quotations from famous and familiar people. But it's far from finished. It really helps to get feedback - I've added many quotations that were asked about in the forum.
So, if your favorite author is missing or underrepresented, please do let me know! (and ideally, do my work for me by contributing some quotes.