From "Meausuring Color"
"... Because these ratios of the tristimulus values are incorporated as cube-roots, there can be no chromaticity diagram associated with the CIELAB space, and therefore no correlate of saturation. This is because the definition of a chromaticity diagram requires that any mixture of two colours falls on the straight line joining those two colours: in CIELAB space this line would not be straight because of the cube-root transformation."
I don't have any exact idea ...
I think that ... top views in CIELab look like the projections of the perceptually uniformed and so deformed RGB cubes ... preserving their hexogonal structures.
It is like getting the HLS represantation of a RGB cube by projecting it on a plane as a hexagon,