Camera is better at seeing than our eyes. According to http://www.cambridgeincolour.com/tutorials/cameras-vs-human-eye.htm astrophotographers have estimated that human eyes have a ISO rating of 500-1000 after being properly acclimated to dark conditions. The example photograph has used something like ISO 3200.
Cameras can also take longer exposure (=gather more light) than human eye. Our eyes don't see any more details after staring at a subject 10-15 seconds.
Some astrophotographs can have over 24h worth of light collected (usually in 2-15 minute exposures stacked together to make one image), our eyes can't compete with that...
Light pollution is also a major drawback. Light polluted sky is too "bright" to see faint object. Cameras can overcome light pollution by using different kinds of filters to filter it out (f. ex these are all taken from middle of a light polluted city http://www.astroanarchy.blogspot.fi/).