Because the colours are standardised and embed important information. These are tools, not aesthetic objects. A lot of the palette is preserved from paper charts where they need to be readable under red light when underway at night.

For a masterpiece of technical documentation dig up a copy of admiralty chart 5011 for a dive into the colours and symbology used.

It's an international standard so all professionals that read the maps never have to guess what the colors mean. In commercial vessels, you need to follow the Solas standards. This also means that you need certified hardware and software for the chart plotters.