Have you ever looked in the back of an old book and seen advertisements for series and books you've never heard of? One thing's for certain, it's most likely that no one's reprinting those.
So, why are the Oz books still being reprinted?
Looking at the Oz books critically, even ardent Baum fans have to admit that the guy wasn't perfect. The Wonderful Wizard of Oz contains what feels like a large anticlimax as Dorothy and her friends have more adventures on their way to Glinda's. While it's a charming piece on its own, the Dainty China Country doesn't even fit with this anticlimax. One can view these adventures as Dorothy's friends having to prove to themselves that they can think, love and have courage, and while the Fighting Trees allow the Tin Woodman to have the heart to fight them so his friends can pass, the Giant Spider gives the Lion a chance to prove his courage and the problem of getting across the Hammerheads' mountain is solved by the Scarecrow (plus, after he got his brains, he acts pompously, and during these adventures, he gets beaten twice by the Trees and the Hammerheads); the China Country offers no such development, other than Dorothy's friends proving their resourcefulness, which has already been done.
I also note that Baum doesn't really do character development, and when he does, it's over the course of several books. Dorothy goes from crying about not being able to return to Kansas in Wizard to basically saying, "Aunt Em and Uncle Henry won't worry if I'm not gone for too long" in The Road to Oz. The Nome King goes from a tricky gambling king with an amiable disposition in Ozma of Oz to a tyrant in Tik-Tok of Oz. The Tin Woodman takes years to remember to find the Munchkin girl who he was going to marry, realizing that his heart is not loving but kind. And some cynics note Ozma's diplomatic acts throughout the series being rather questionable. (Fellow blogger Mari Ness noted this as "Ozma Fail.")
In The Patchwork Girl of Oz, Baum literally changes the main cast halfway through the book, something that seriously isn't done. And, oddly, while Dorothy refuses to move to Oz until Aunt Em and Uncle Henry are also allowed, no thought is given to the parents of Trot and Button-Bright when they take up residence as well.
Why, when the Oz books have issues, are they still around? There's the popularity of the MGM movie, but a famous film doesn't always assure that the original book will be widely available—at least, not as widely available as The Wonderful Wizard of Oz—much less its numerous sequels. (Go to a used DVD store. Locate a copy of Willy Wonka and the Chocolate Factory or Charlie and the Chocolate Factory. Found it? Now go to a used bookstore and find the book and Charlie and the Great Glass Elevator. Having the same luck?) Of course, Baum's books being public domain is a large factor in their availability, but copies of J.R.R. Tolkien's The Hobbit and The Lord of the Rings and C.S. Lewis' The Chronicles of Narnia are also just about as easy to come by, and their copyrights are still active.
Actually, it's a good thing I mentioned Tolkien and Lewis there. During the times that their works were first published (particularly Lewis), it was widely believed that children needed to be grounded in reality and thus, fantasy in all its forms was harmful. C.S. Lewis wrote various essays about the need for fantasy, and Tolkien produced a fine piece called "On Fairy Stories."
In the United States, the Oz books were under fire from librarians who felt that fantasy should be avoided. It wasn't just any idea about literary quality, there were financial reasons as well: about the time this began to happen, there were at least 38 books in the main series. Long series were avoided by libraries, and the more popular they were, the worse it was perceived to be. Not only would there be the cost of stocking copies of the book, there would also be the cost of replacing them should they wear out. (Not to mention the possibility of lost or stolen books.)
People like Martin Gardner, C. Warren Hollister and Daniel P. Mannix argued for the value of the Oz books. Despite lacking certain literary qualities that critics deemed important, they argued that the inspiration offered by fantasy was actually quite beneficial to young minds, and that a series like the Oz books that offered a cast of positive and miraculously believable characters were a good influence on children. General opinions changed, and the children who did enjoy the Oz books when they were released eventually took over.
And yes, they have a point. Hollister said the Oz books had a three-dimensionality. Part of this is due to what critics would call errors in the Oz books: the seemingly pointless side adventures helped the stories feel more real rather than a neatly polished narrative. The wide range of characterization helped children believe in them, even if they knew they were just reading a fairy story.
Take this in comparison to J.R.R. Tolkien who also included easily removable adventures (Tom Bombadil, the Barrow-Downs, the Scouring of the Shire), yet his narrative, though quite enjoyable on its own, is not the same as Oz. Oz is joyful, while Middle-Earth can get so gritty, it feels dry at times.
Why are the Oz books still widely available when so many other old series from the beginning of the 20th century are forgotten? They sell. Why do they sell? They have a seductive quality that keeps the reader coming back for more.