Why can't Disney renew Mickey Mouse?

Could you elaborate on the reasons behind Disney's inability to renew Mickey Mouse's copyright? Is it due to the fact that the character's creation dates back decades ago, and thus, the original copyright has expired? Or is there some other legal or contractual obstacle preventing Disney from maintaining its exclusive rights over the beloved icon? It would be intriguing to understand the complexities involved in this matter and how they impact Disney's business strategy and its ability to leverage the brand's immense popularity.