In Tron Uprising we find that Tron survived Clu's attack, although he is severely injured. He is hiding far from Tron City. Presumably he knew that Clu has failed to capture Flynn, and that Flynn failed to exit the Grid. So why wasn't he out looking for him? Why was he hiding in his cave (much as Flynn was) ?
TLDR; He cannot for two reasons: he is too damaged, and he didn't know where to look for Flynn.
First, as we see in TRON: Uprising, Tron relies on a healing chamber to keep himself alive. That prevents him going on a trip to find his old friend.
In TRON: Uprising, Tron resented no longer being able to fight as well as he used to, due to extensive injuries that repeatedly opened again unless he was able to constantly access a healing chamber. Despite this, Tron was a skillful and deadly fighter, able to dodge Beck's attacks with both his hands behind his back and roundly defeat one of Clu's top enforcers. It is also worth noting that Tron took a direct hit from Clu's identity disc but survived. However, his need for a constant power source and regular access to a healing chamber - without which he would derez - prevented him from being able to retake the Grid himself.
- Tron's Tron Wikia article Skills and abilities section
Second, Flynn hid himself very well, and he did it for a long time. If CLU, having had access to almost all the Grid's resources, didn't find him in 1,000 grid relative years, what are the chances of a lone program finding him? Since Flynn thought Tron was derezed, He made no effort to find him on his side.