What critical factor makes the North America Nebula appear faint to the naked eye despite having a total visual magnitude of 4.0?

Answer

Low surface brightness

Although the integrated magnitude is bright, the light is spread thin over a very large angular extent, resulting in incredibly low light hitting the eye per square arcminute.

What critical factor makes the North America Nebula appear faint to the naked eye despite having a total visual magnitude of 4.0?
astronomynebulabrightnessNorth America Nebula