I have decided to compile a list of plausible reasons why Yori was not in TRON LEGACY. Some of reflect rather negatively on Disney and Jospeh Kosinski, while others leave them a way out. These are nothing but speculative simple possibilities which may or may not be the truth.
1. The idiots didn't watch the first Tron movie, and they have no idea who Yori is.
2. They forgot. While they were busy figuring out how much crap in the movie will be 3-D and how to do it, they almost forgot about Tron himself, Bruce Boxleitner as well.
3. They decided that Yori would get in the way of the other female characters in TRON LEGACY, and in their uncreative ignorance assumed that having any female character in a film means that she must be a sex-symbol and they couldn't figure out how to work a third sex symbol into the script.
4. The original script for Tron Legacy didn't include Flynn, Tron, or even Alan, and if they hadn't gotten Jeff Bridges on board, almost nothing from the original Tron would have been in the movie. You're lucky to have gotten that.
5. They have plans for Yori to appear in the third film, she will be integral to the mystery of the ISOs.
6. If Yori had appeared in TRON LEGACY, it would have required real writers who have the capacity for imagination, and the thought was, if TRON (Rinzler) is evil, how will Yori fit into the equation? Since they couldn't figure it out, they just ignored it, hoping the audience would forget about Cindy Morgan and the original movie.
7. Yori will be in TRON 3 because who else will help Rinzler (TRON) when he washes up on shore of the Sea of Simulation, and pump the plasma out of his lungs?
8. Yori was actually in the TRON LEGACY script originally, and would have been the part of Castor, she operated a night club, and Quorra decided to send Sam to see her because she knew Yori could help him. Rinzler and Clu show up, and Clu tortures Yori in front of Rinzler to find out where Sam is, this was the original motivation for Rinzler's turn to the 'good side.' However, they decided that they wanted something more 'fun' at that point in the movie, and that torturing a digital character, and setting up motivations for Tron in the movie was 'just stupid' so they opted to create a new character, and that also gave them the opportunity to have Gem show up again for no reason.
9. They simply didn't have enough time or money to computer generate any other characters besides Jeff Bridges. Bruce Boxleitner's head was pasted on the guy who played Rinzler to do the scenes with the original Tron, but he wasn't actually there filming them, and this was last minute, which is why they fogged up the lens and he's only in it for 10 seconds. They ran out of time, and money, otherwise the MAIN CHARACTER, Tron would have actually been in the movie more. Therefore, any other characters, such as Yori, were simply cut out altogether, since there was no way in which they could have done the effects in order to get the movie done.
10. The director decided that Yori was like a James Bond girl, and that Tron should have a different chick every movie, he has no understanding of the original Tron movie, which is why he opted to barely even have Tron in the movie called TRON LEGACY at all. If it wasn't for Jeff Bridges and Steven Lisberger, Kosinski would have made a movie called 'TRON' Legacy which would have little or no resemblence, nor anything to do with the original film.