Actually Cinepaint was used for a quite a while in the movies where it served it's purpose rather well. There was a point in time where studios I believe were funding the development of it because commercial offerings at the time were poor.

The list of movies is listed on the Cinepaint wikipedia page, and I'm not sure that's all of them. It seems like the last movie listed was back in 2003.

https://en.wikipedia.org/wiki/CinePaint

As to why it's not more popular? I'm guessing because studios switched over to commercial offering once Adobe and other software houses got their act together, and provided software with more features and support than Cinepaint.

Photoshop seems to have had high bit depth editing for at least the last decade, and possibly longer.