... Fox would love “The Peanuts Movie” to become a franchise, but the studio only has the rights to one film and Jean Schulz [Charles Schulz's widow] said she is no rush to produce a sequel. “This one took eight years, so maybe we’ll talk again then." ...
So the question is, why wouldn't Rupert and his troops have the rights to several more? I suppose the answer is, you negotiate what you can negotiate, and hope for the best. Mr.s Schulz appears to have had some leverage.
And now she has more.