I wonder why manufacturers stay within the 300cd/m2 limits,
Those kind of limits are non damaging to the eye, yes, but when you're in a room where the sun shines in, or when you're working in a very low light environment, those displays just don't work!
They either give off too much light in low light, or are too dim in bright sunlight (looking grey).
Mitsubishi, originally a car manufacturer btw, can have all engine they want, if the display is used in a sunny environment (office with wide windows), the display will look grey, dim, and colors might look washed out!