It's not obvious whether any of the comments here that "they didn't feel it was worth it" are actually informed, or that's synthesis (guessing).
> “We had to have some very, very smart people at Pixar go back in and write some software and figure out a way to make it so that those files would render on our current computers,” he said.
>It took four months to resurrect the old data and get it in working order. Then, adding 3-D to each of the films took six months per film.
https://www.nytimes.com/2009/10/04/movies/04murp.html
Sounds like getting the render to work was about as much effort as converting to 3D.