The Naragansetts ("Excursion") were actually around years before the "Retlaws" were taken out of service (1958, with the #3 "Fred Gurley"). With the arrival of that Primeval World diorama from the World's Fair "Magic Skyway", Disney apparently ordered "Holiday Blue" and "Holiday Green" based on the converted 'Freight' consist. The inward-facing bench seating was also copied on the operation at Animal Kingdom in Florida, for reasons (most likely) of keeping you from seeing what they didn't want you to!
As to why the Excursion wasn't copied two more times, I don't know; but I sure wish it was. Heck of more comfortable, and apparently Florida and Tokyo liked the design!