Why is VGA connector has only one pin for red color data but is composed of 8 bits?
A VGA signal cable uses analog signals for R, G, and B. H-sync and V-sync are... digital sync pulses.. but the color channels need to be converted into a digital signal through a sampling process to be fed into an LCD.

Traditionally these signals were fed into a CRT with a red, green, and blue cathode ray, and so these signals were simply modulated into the beam intensity of each.