The simple answer is that the cone cells in the human eye are mostly non responsive at low light level therefore you do not see the colours present, the rod cells which are what you utilise for low light vision only give you grayscale vision.
The threshold will be different for different people and is affected by age etc. so some people do see some colour in nebulae etc where others, particularly older eyes have to contend with shades of grey
As for the reasons some are drawn to AP ???? that's anyone's guess.
I'm into "astrophotography-by-proxy" that is I love seeing the results of the hours of effort the AP afficionados put into gathering data but prefer my own time to be with my eye to the eyepiece.
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