Because a mobile phone screen makes images by emitting light, but it cannot compete with sunlight. If the ambient light is brighter than the light the screen can produce, then the screen is hard to see.
The best color display I ever had was a Sony Clie PDA. It was a color LCD, easy to read in full sun (actually the brighter the ambient light, the better) and in dim light or darkness the backlight was excellent and evenly distributed. It made me wish all mobile phones used such displays.
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