It seems to me there's a fundamentally different way we experience film vs a stage show. For example, the camera.
Russell Crowe's singing wasn't great, but because he could express he character in close-ups, like he was singing right at us from three feet away, it was far more acceptable than it would have been in a stage show, where he could never get away singing that softly.
Also, movies are experiences as a series of scenes, when a stage musical is a series of musical numbers. IIRC, in the stage Les Miz, the young students sing the rousing anthem "Red and Black" followed almost immediately by the rousing anthem "Angry Men."
On stage it's two big numbers.
On a screen it's one really really long scene.
On top of that, the film could stage a huge riot in the streets,that couldn't be staged, one that serves as a swell point to sing the "Angry Men" song, so splitting those numbers and moving "Angry Men" to the next day IMHO worked quite well.