Fibers are not transparent enough in visible. I found about 10dB/km (to be compared with 0.2 dB/km around 1550nm) [0]. This means that after every km the light intensity is divided by 10, which is completely impractical for telecommunications.
That depends on the application, the standard for short range (<100m -- fine for most anything that doesn't leave your building) is already to use 800nm.
