From Wikipedia on the Bayer Filter:
"Bryce Bayer's patent (U.S. Patent No. 3,971,065) in 1976 called the green photosensors luminance-sensitive elements and the red and blue ones chrominance-sensitive elements. He used twice as many green elements as red or blue to mimic the physiology of the human eye. The luminance perception of the human retina uses M and L cone cells combined, during daylight vision, which are most sensitive to green light."
The Foveon sensor does sample RGB at every pixel by using 3 layers, but it is only used in a few cameras.
50% of the sensor sites are green (25% each for red and blue), so there is more signal given the same amount of noise.