Because the two bands of red and blue is traditional. As you couldn't/wouldn't stitch together dozens of small pieces of coloured fabric to make a union jack jersey 50-100 years ago a simpler design suitable for manufacture has to be found. The British Olympic Teams kit was regularly the same design around the time National Cyclists Union and the British League of Racing Cyclists amalgamated into BCF so iirc that's where the design comes from.
At the 1932 Olympics, the British teams wore kits with three horizontal bands. The two bands of red and blue seem to have been introduced for the 1936 Olympics.