The cold facts on Disney not moving forward (at this time) on a third TRON film are these:
$180 Million budget
$150 Million in advertising
$400 Million gross.
But beyond the math, the other issues was lack of interest. TRON: Legacy, the 2010 follow-up to the 1982 classic, was a visually pleasing film from Joseph Kosinski (helming his first film after a career in doing CGI commercials) with a better than average techno-score from Daft Punk. But unlike it's predecessor, the new TRON film scored badly with critics and film audiences. And in the five years since its release, interest in a third film waned. In the end, it did not become the cult classic the original did. And in many ways, that's what kills a franchise more than anything, no one really caring.
Sure Disney never officially announced a third film, but it seemed the (contactually obligated) actors where ready to assemble (with director Kosinski) in Vancouver where rumors were piling up that this is were Disney was hoping to make a much cheaper sequel. But while the hardcore fans held out hope, it's a good guess the decision to not move forward after all was made by the lukewarm performance of another expensive film, Brad Bird's $190 million (well, $280 with advertisement) Tomorrowland, which opened May 22, 2015 with $33 million weekend (and took in $42 million during the three-day holiday weekend that followed). While that was fairly on par, it only scored a bit better than Pitch Perfect 2, which cost only $29 million and has taken in $228 million to Tomorrowland's $133 million so far in the same number of weeks out. In Tomorrowland's defense, of course, its an original film and not a sequel or a reboot, which does make it's success an uphill climb. And like TRON: Legacy, Tomorrowland has its fans (mostly, it seems, Disney followers), but like the former, the film has generated a mixed bag of reviews. So we come back to the same problem that plagued the TRON sequel: visually pleasing, but empty of characterization.
This is Hollywood cynicism of tent-pole film making in a nutshell: The philosophy that these types of films only need to pleasing to the eye with a electronica soundtrack done by a popular group while ignoring all aspects of internal logic, storytelling, creating interesting and believable three dimensional characters.
Plus you don't hand over what was once a very influential and original film to someone who's whole career is based on making CGI beer commercials. Yes, Kosinski was cheaper than hiring a director with a track record, but historically giving a large budgeted film to someone who has little understanding of how to use that budget is like expecting congress to live on minimum wage.
I don't think, however, Disney is done with TRON. It has spent a boat load of money on it and will eventually figure out a way to recoup their costs -they're very good at that. But maybe they should wait a bit, figure out a good idea (reboot or move forward from the second film) before committing to doing another film. Plus, it doesn't have to be that expensive of a prospect either. But in the end, like all films should, the script is the most important thing. If you have a weak story, no amount of visual flair, CGI, Daft Punk music or nearly naked Garrett Hedlund can make a TRON film successful.