This is due to Nyquist–Shannon sampling theorem which says that you can reconstruct a signal only if you have twice the sampling rate. It's important in signal processing.
For example, in hi-fi audio you have a sampling rate of 44.1 kHz or above. This is to ensure that you can reconstruct 20 kHz which some humans can still hear, not me anymore :)
It is also the reason telephone sounds crappy. It is sampled at 8 kHz which means that it cuts all frequencies below 4 kHz. But it's a good trade-off for the bandwidth you get.
The same principle applies to the images captured by a camera.