It seems to me that there are a couple factors at play:
- children's fiction specifically gets altered to keep up with the times, because children are thought to be particularly vulnerable to certain content in media. Think of how many Disney movies are based on fairy tales that were a lot more violent in the 18th century.
- In an era where every media property is a potential multimedia franchise, you're going to see companies that want to promote older works without signing on to everything contained therein.