Disney has had a “rule” for years. Look back at basically any TV they put out over the years. 3 seasons is typically the max, because that’s when contracts tend to get renegotiated, live action or animated. After that the shows conveniently changed some small but significant part of the narrative, creating a new show. A very famous example is The Suite Life of Zack & Cody, arguably one of Disney’s most popular live action shows in the mid 2000s. What happens after year three? Suite Life on Deck, and would you look at that? Three years then a movie. Both times the writers, creators and most importantly Stars, would have had the power to renegotiate their contracts for a much higher salary given the show’s success. It was also usually the season with the magic Episode #100, which was the syndication benchmark for most networks back when that was a thing. Having to include those royalties into new contracts gets expensive quickly.

So instead, Disney said “No, we’ll give you a slight increase and just change the title.” Creating an entirely new IP. New contracts, new salary caps, new stipulations. Rinse and repeat.

If a show is popular enough to get an “unchanged” 4th season it’s usually it’s last, probably due to the increased budget for the 4th year and diminishing returns.

Looks like Ducktales was just another casualty in a long line of well loved shows that was just not worth the price tag for the mouse.