If I am correct, "System WHY" is not a term used by Geobra, but invented by Collectors (tahra, probably!), in parody of 'System X,' as a protest about building parts which can only be combined with parts from the same set, so there is little or no scope for creating your own buildings with them. They are sometimes joined with yellow and red connectors, as in System X buildings.
For anyone unfamiliar with English, the letter Y is pronounced the same as the word 'why,' so it's a clever parody! Why not produce more versatile pieces?