In any art form there’s the art, but also the craft. You can have the best idea or theme for a painting, but if you don’t understand the craft of mixing paints to make the right shade, you probably won’t become a well regarded painter.

As a composer you can have an amazing idea or even an amazing melody, but if you don’t understand how to write it in a way that real musicians can play, it won’t be a successful musical work.

So practically that means things like having enough rests in the parts so players aren’t playing too much without a break. Writing parts not just in the high register which may be more physically taxing. Not having a melody in one clarinet that’s accompanied by the entire brass section, because it wouldn’t get heard without microphones and amplification.

There are plenty more examples I could come up, with but those are a few that don’t work well for live performance, but are easily overcome in the world of studio recording where you can adjust the balance any way you want, or stop and re record passages at will, or save the upcoming difficult passage to record tomorrow when everyone’s fresh again.

That doesn’t mean that film scores aren’t great art. It just means that they weren’t designed with the same “live performance” type requirements in mind. The craft of film scoring is a bit different than the craft of writing a symphony.