This is due to legal issues from the voice provider. Originally voice actors and actresses were nervous about letting people know they were the voice actor because people could " steal " their voice. This is still a major issue among the English Vocaloids today.
There are actors who simply don't want to be revealed for their own personal or privacy reasons. However, the possibilities behind this are speculative.
Additionally, young voice providers are not revealed because they are under the age of 18. Also, some voice providers are not revealed because of their country's legal issues that prevent their providers from being revealed.