The answer is rather a simple and sad one. To Disney, their television cartoons were meant to be forgotten. Don't get me wrong, many of them were excellent products with strong characterization, snappy writing, good stories, and decent quality to the animation. I grew up with these shows and then some.
However, to Disney, they are not what the company would consider "the franchise." Sure, DuckTales (1987) and (2017) feature the Duck family rather heavily and have very recognizable Disney characters in them like Donald, Uncle Scrooge, and Huey, Dewey, & Louie. But Disney does not seem to find these things bankable enough to carry the company, ironically. The television shows that Disney makes itself and/or acquires from outside are often very good. But they have a lifespan, and the harsh fact is that a TV series can alienate people who aren't familiar with it because, unlike a Disney Princess movie, it often has many different stories and characters, weaving itself over the life of the show. But hey, everyone's seen Frozen. And the film tells you everything you need to know about Anna and Elsa. There isn't going to be an issue where you haven't seen this particular episode of a long-reaching series and thus are unaware of vital information about a character.
Disney's TV series were meant to be consumed and then slowly fade into obscurity. Remember, most of the shows we're pining for pre-date the internet. The way fandom worked in the 80s and 90s is very different than today, people couldn't so easily share their love of things with one another. There were no forums and chat servers and the like.
As a cruel example to the way Disney handles movies versus television, consider the already aforementioned Star Butterfly. As dynamic and wonderful a character as she is, and despite being on Disney XD and Disney Channel, she will never be a Disney Princess. Why? It's a question of citizenship. Star is not "naturally born"--she was not created by Disney but rather purchased. Disney is happy to use her to front its channels and further its domination of family programming, along with its other hit shows, but there is never going to be a Disney Princess figure of Star. As it is note that the merchandising for very popular Disney shows like Star vs. The Forces Of Evil and Gravity Falls is very limited, because Disney doesn't want to draw too much attention away from Mickey and the rest of the "franchise."
All this said, I think worlds with some of these TV shows would be a wondrous thing, and I'd love to see them, but I wouldn't hold my breath.
