It opens and closes in Black and White, of course - opening images (The Set-Up) contain all the characters who will re-appear in the contrary (colourful) world. The good companions, and the genuinely evil witch, as well as the incompetent but benign wizard. Also the themes of Courage, Brains and Heart get introduced, but I guess the Theme Stated must be Somewhere Over the Rainbow (Aunt Em tells her to go look for a place where there is no trouble)...
The Catalyst arrives bang on page 12 - as Toto escapes back to Dorothy but she knows her Aunt and Uncle will give way to the evil one, so has to choose to run away.
Now we get the 'debate' section as she meets the Professor who tries to get her to turn back from such a decision, but by page 33 we reach the Break into the Second Act ( and into colour, of course).
I am not sure about the B Story (in Snyder's model) but Dorothy meets and builds up relationships with three 'helpful creatures' and they merry along the road to the MidPoint, which uncannily arrives (as Snyder says it should) on page 65, exactly half way through - where Dorothy sees the Emerald City, but has fallen asleep in the Poppy Field!
The baddies then do close in for quite some pages, and their first meeting with Oz turns into a 'False Victory' (All Is Lost) as he sets them a daunting task that throws them back to the baddies lair (the Dark Night of The Soul).
The Break into Act Three seems like the moment when Toto escapes and (like Rin-Tin-Tin) goes to find the Helpers and lead them to rescue Dorothy, by using the aspects of themselves they didn't previously believe in.
In the Finale they overthrow the witch and all return to find the Wizard, who grants them their (non-magical) rewards. He then appears to help Dorothy get home with his hot air balloon, but it escapes (false ending) but Glinda arrives to tell Dorothy she has the power within herself to return...she just has to want to "There's No Place Like Home".
And the final image, of course, in Black and White, she is home, but hasn't had to say goodbye, as it turns out that Zeke (Lion), Hunk (Scarecrow) and Hickory (Tin Man) are still there, in human guise, and even the Wizard/Professor pops his head in...
Yup, I can see why it works.