This also applies to the later movies and DS9, but I was always curious why they decided to change to those dull gray uniforms with the colors only showing up on their collars and their sleeves.
Probably the same reason
Generations used dimmer lighting than TNG, and the same reason the TOS movie uniforms used more subdued colors than the TV uniforms. TV shows are made to be watched on relatively small screens in people's living rooms with other sources of light around, whereas movies are made to be shown on huge screens in enclosed chambers that are otherwise extremely dark. The kind of bright colors that work well on TV would be too garish for movie screens, so feature-film cinematographers, costume designers, set designers, and the like tend to favor more subdued color and lighting. Or at least, that was the case back when the movies in question were made, before giant flatscreen TVs were common and before movie theaters began using digital projectors. The difference is probably a lot less now than it was back then.