The Wizard of Oz was not, as many people believe, a total flop. Indeed, it made around $3 million at a time, second that year* only to Gone with the Wind (which was an insane, off-the-charts success). The problem was that it also cost almost $3 million to make, leaving the studio with a loss on the movie when theater shares and marketing were factored in. While it was enormously popular with the critics and very popular with the audiences (who only paid an average of a quarter per showing, the equivalent of a little over $4 today), ongoing production costs would not have improved much, and so there was no financial reason to continue to turn the other books into films.

Source: Susan King, "How did 'Wizard of Oz' fare on its 1939 release?," LA Times

*Edit: This may not be quite the case. Box office records were not kept especially accurately at that time, and apparently were estimated based on how much business went to a given movie compared to an "average" movie. In any case, it was absolutely in the top 10, but apparently ranged somewhere between second and sixth overall for 1939. Not a bad showing in any case, but maybe a little like Waterworld, which came in 12th for 1995 movies but lost money at the box office.