About one in twelve men has some kind of colour weakness. If you're relying on the ability to distinguish between colours on hue or saturation alone, you're making it difficult or impossible for one in twelve men to use your product.
It's not that hard: if you need to distinguish a small number of colours, make them sufficiently different in luminosity: if you wouldn't be able to tell the colours apart on a greyscale photo copy, someone will be unable to use it. If you need to distinguish between a large number of colours, use additional clues (e.g. labels, patterns, etc) other than the colour itself.
It's not "not easy to distinguish between all colours". It's impossible to distinguish between some colours, making the map itself meaningless. This is akin to having a heatmap where the "hottest" and "coldest" areas are indicated with the same colour: the result is useless unless you're specifically interested in "average" vs "extremes" (with no regard for direction).
It's not that hard: if you need to distinguish a small number of colours, make them sufficiently different in luminosity: if you wouldn't be able to tell the colours apart on a greyscale photo copy, someone will be unable to use it. If you need to distinguish between a large number of colours, use additional clues (e.g. labels, patterns, etc) other than the colour itself.
It's not "not easy to distinguish between all colours". It's impossible to distinguish between some colours, making the map itself meaningless. This is akin to having a heatmap where the "hottest" and "coldest" areas are indicated with the same colour: the result is useless unless you're specifically interested in "average" vs "extremes" (with no regard for direction).