To summarize, Disney began work on a sequel to Tron: Legacy immediately. Legacy was meant to kick off a larger universe of movies and TV. As time went on, other expected hits flopped, and Disney acquired Star Wars and made it their new darling, the Tron sequel got shelved.
Jared Leto, who was going to have a minor role in Tron: Ascension, pushed to have a new movie made anyway, making his character the star. The result is that what was more of less going to be the third act of Ascension was fleshed out into a whole movie minus Sam Flynn, Quora, and Tron.
Imagine if Return of the Jedi was never made. Instead, the third act of the movie, the attack on the second Death Star, was turned into a whole movie but the continuation of the plots involving Han Solo, Darth Vader, and Luke Skywalker were completely cut out. Then Wedge Antilles was made the star of the movie and he personally confronted Chancellor Palpatine and saved the galaxy. That’s pretty much what has happened to the Tron franchise. :(
I have a group of friends that are fanboys of Tron in levels that I still don’t understand. They were to the premiere cosplayed as the characters* and still today think that they are a conspiracy to bad mouth the movie and kill the franchise.
* one of them is a professional cosplayer and was paid by a shopping mall to make some art to decore the theater for the premiere, but the other ones are just fans.