That's only for the NG models 600-900. The Classic 300-500 has a different criteria (what does one call the 100-200 now? Dinosaur?).
That is how Boeing determined it. As pilots, we follow the guidelines that come from the manufacturer. It has been decided we don't have to know the reason, just do it! I guess the reason is that it has been determined that the NG wing, which is markedly different to previous versions, is not significantly adversely affected by icing in normal landing configurations.