How can pixels display brown, while RGB led's lights can't?

Brown is a very weird colour. It's a colour of context. It doesn't exist otherwise. We perceive dark shades of orange as brown, because we decided to separate them as separate colour. You can only see brown if you can see surrounding area, context, in which that colour is displayed. Monitor can very well control amount of light emitted and also display almost perfect black next to a colour. To see brown, you must display brighter surrounding what is totally possible on monitor. LED can decrease its brightness, but it is emitting light in a way that can't display dimmed orange fragment surrounded by brighter light, hence your brain can't perceive brown in such setup and will always see shades of orange at best.