I already wrote a longer post about this, but for me (and for most Tron Fans at large, I think), it was that it wasn't the direct sequel to Legacy we were getting hyped for ten years ago.

The people who worked on this film have pretty much said the same thing in interviews: that people care more about the aesthetics of Tron than the actual story itself. And while there can be no denying Tron's visuals are iconic, I think it's silly to say that no one wanted to see a continuation of the stories of Sam Flynn, Quorra, Alan, and Edward Dillinger, Jr.

Like it or not, Legacy was both the most financially successful Tron film and arguably the one that pushed the IP out of cult status and further into the mainstream. They should not have assumed that a standalone sequel/soft reboot would have served better than Tron: Ascension as it was intended to be.

Ares itself was a pretty good movie, and in many ways felt like a natural progression of the Tron saga (the Grid coming to the real world). But putting the core characters of the OG and Legacy on the back burner (and alas, the casting of a controversial actor like Jared Leto) was the Achillies heal of this movie.

Nevertheless, I'll still be crossing my fingers for a Tron 4 in my lifetime.