Because Peter Pan Peanutbutter has been has been around since 1928 (actually, it has been around longer, but was renamed Peter Pan in '28). That's the same year Walt Disney developed Mickey Mouse and 6 years before Donald Duck. By the time Disney released their film version of Peter Pan in 1953, the peanut butter brand had been using the name for a quarter of a century.
Peter Pan was a well known children's book well before Disney animated it. IMDB lists the first Peter Pan film as released in 1924. The 100th anniversary edition of the book was released in 2003, which means Walt was all of two when the book was first published. Peter Pan pre-dates Disney.
Donald, on the other hand, was created by and for Disney. As far as I know, Disney has always owned Donald. Thus products with Donald's name and/or image must be licensed by Disney. I don't think Sandy Duncan needed Disney's stamp of approval everytime she played Peter Pan on stage. Disney own's their animated film version of Peter Pan, not the brand as a whole. At least as far as I'm aware.