Why does the chromaticity diagram look like that?

In my opinion, plotting chromaticity on a Cartesian grid — by far the most common way — is pretty misleading, since chromaticity diagrams use barycentric coordinates (and to be clear, I blame the institution, not the author). The effect is that the shape of the gamut looks skewed, but only because of how it's plotted; the weird skewedness of a typical XYZ chromaticity diagram doesn't represent anything real about the data.

Instead, a chromaticity diagram is better thought of as a 2D planar slice of a 3D color space, specifically the slice through all three standard unit vectors. From this conception, it's much more natural to plot a chromaticity diagram in an equilateral triangle, such as the diagram at [1]. A plot in a triangle makes it clear, for instance, that the full color gamut in XYZ space isn't some arbitrary, weird, squished shape, but instead was intentionally chosen in a way that fills the positive octant pretty well given the constraints of human vision.

[1]: https://physics.stackexchange.com/questions/777501/why-is-th...