Disney is a film production company and has to make sure they can protect the rights of any products they create, for their toys, products and other licensing. Disney didn't use any of those previous names because they belong to other writers, other works and they did not want to have to license those names from those respective writers.
Since Baum has never named The Wicked Witch of the West, Disney took creative license and gave her a name and a backstory suitable for their needs. Disney and Warner Bros. are already in a legal battle because of the 1939 film, the Wonderful Wizard of Oz. Note for example the lack of Ruby Slippers, a trademarked icon for the 1939 Wizard of Oz and as such was not replicated or even noted in Oz, the Great and Powerful.
As recounted in the Hollywood Reporter:
L. Frank Baum first told the story in 1899 in his classic novel, The Wonderful Wizard of Oz. Since the book was published more than 110 years ago, one might assume that anyone has the right to make a film featuring public domain characters such as the Scarecrow and the Cowardly Lion.
Why is this newsworthy? Well, Disney's coming reboot of the story, directed by Sam Raimi and starring James Franco, is titled Oz, the Great and Powerful. Warners filed its trademark registration only one week after Disney had filed its own.
On Wednesday, an examiner at the United States Trademark Office suspended Warners' trademark attempt because Disney had come first. But that's not the end of the story, if a recent 8th Circuit Court of Appeals decision and lots of activity at the Trademark Office are any indication.
Although Baum's book and accompanying illustrations are in the public domain, judges at the 8th Circuit last year decided to give Warner Bros. "character protection" under its copyright on the 1939 film starring Judy Garland. In the case, which concerned a company that attempted to sell film nostalgia merchandise, the appellate circuit ruled that it would be hard to visualize these characters without watching the movie, even if one had read the book beforehand. -- Disney, Warner Bros. Fighting Over 'Wizard of Oz' Trademarks (Exclusive) -- Eriq Gardner
Since Disney and Warner are both in court over the use of L. Frank Baum's works, it would make sense they did not want to muddy the waters further by interacting with any other previously existing products whose rights might be more easily defended in court.
