Christopher Tolkien has always been the roadblock to theme park rights; heck, pretty much any licensing of the properties. I’m still floored the movies got made.
That having been said, something changed relatively recently, and I was equally floored at the Amazon series news. Which leads me to believe that the theme park rights have been, if not actively shopped, at least available to be discussed.
I’m guessing Christopher, now in his mid-90’s, has lost his grip on control over the estate and the younger family members are looking to cash in.