I suspect its because when conditions are right to actually see color in deep-sky objects, its confounding that it doesn't look the same as the pictures. Especially if seeing the colors with your own eyes feels like a transcendent experience.
I've only experienced dramatic color from deep sky objects a few times (the blue of the Orion Nebula vastly outshines all the other colors, for instance), and its always sort of frustrating that the picture show something so wildly different from what my own eyes see.
There's a good chance the real problem there is limited gamut on the screen, and with the right viewing method the RAW photo could look much much better.
I've only experienced dramatic color from deep sky objects a few times (the blue of the Orion Nebula vastly outshines all the other colors, for instance), and its always sort of frustrating that the picture show something so wildly different from what my own eyes see.