Familytime
Generally, for content such as Hazel and other vintage TV shows, Roku doesn't have direct license with the content owners. Often, it's an intermediary party that has the rights and enters into contracts with other services, to include Roku Channel as well as other apps/services.
Sometimes, content may not be available for copyright reasons. Often, that has to do with copyright lapsing on content, or content produced by another company that is in a questionable status. There are many many reasons why certain content isn't available. Sometimes, the original recordings no longer exist. Heck, NASA erased the moon landing footage in order to save money on tapes. Many production companies erased copies for budgetary reasons over the years.
The reason there hasn't been a response is often because no one knows the answer. I don't, but I know how some of the stuff works, and what I've listed are often the reasons. It usually boils down to Roku (Pluto TV, Xumo, Tubi, others as well) offering what they can get, with no actual explanation given to them of why more isn't available.