why not just use rgb distance?

[universewill]

Why not just use rgb color space to calculate color difference?

[KelSolaar]

The RGB colour model is simply not perceptually uniform, so colour differences in any RGB space does not correlate well with the human perception of how different the colours effectively are. This is the core reason why perceptually uniform colourspace such as CIE Lab and the Delta-E 1994 and 2000 metrics.