Here's why classic Western series Gunsmoke was canceled after 20 years and didn't receive a finale episode. Before it became a TV show, Gunsmoke started life as a radio program. While John Wayne was once reported to have been in contention for the lead role of Marshal Matt Dillon, this was eventually debunked, as it was unheard of for a movie star to switch to a TV series during this era. Wayne did, however, record an introduction to Gunsmoke's first episode in support of his friend James Arness, who did take the Dillion role.
Gunsmoke debuted on CBS in 1955, and the series became both a critical and ratings hit. It was also considered grittier and more grounded than the average Western movie of that time. The series would end up running for over 630 episodes, and was once the longest-running scripted primetime series - which is a record The Simpsons eventually broke with season 29 episode "Forgive and Regret."
Arness also appeared in every single episode of Gunsmoke, which also saw countless future movie stars pass through it, including Kurt Russell, Jodie Foster - who actually appeared three times - and Burt Reynolds. Gunsmoke did consistently well in ratings throughout its run, though its popularity dipped in 1967, which saw CBS planning to cancel it. There was a public backlash to this, however, so the network decided to continue producing it. After Gunsmoke's season 20 finale "The Sharecroppers" aired, the cast and crew all expected to be renewed for at least another series, but CBS instead opted to cancel the show and replace it with sitcom spinoff Rhoda.
CBS likely felt Gunsmoke was outdated and wanted to replace it with more contemporary shows, though it was still performing well. This abrupt cancellation meant the crew weren't able to give Gunsmoke a proper finale, however. "The Sharecroppers" was a more comedic outing that was in no way intended as a final episode, which is a point that led to bitterness among the cast and especially Arness. That's a major reason Arness - who starred in the first The Thing adaptation - eventually returned for 1987 TV movie Gunsmoke: Return To Dodge.
While this didn't lead to a TV series revival, Arness did return for another four movies, including Gunsmoke: The Long Ride and 1994's Gunsmoke: One Man's Justice. The latter marked not only the end of Gunsmoke as a franchise - at least for now - it was also Arness' final acting credit. While it's not unheard of for a long-running, hit TV show not to receive a finale - Married With Children would later receive the same fate when it was suddenly axed following season 11 - Gunsmoke ending with no fanfare at all was one of the most egregious. Thankfully, Return To Dodge helped right that wrong.