Why does the chromaticity diagram look like that?

That particular version of the chromaticity diagram makes it look like the colors missing from your display are various shades of laser pointer green as opposed to all the shades of red and blue that are missing because really saturated red and blue primaries are too dim (per unit of energy) to use.

See https://nanosys.com/blog-archive/2012/08/14/color-space-conf...

I learned a lot more about color management than I wanted to know in the progress of making red-cyan stereograms because I found when I asked for sRGB red I was getting something like (180,16,16) on my high gamut monitor which resulted in serious crosstalk between the channels.

Right now I am working with a seamstress friend on custom printed fabrics and I have a flower print where yellow somehow turned to orange in the midst of processing the image and I want to get it debugged and thoroughly proofed before I send out the order... I am still learning more than I want to know about color management.

From your linked article:

>The colored region of the plot, labeled "visible region", contains points in the space representing colors humans can see, while the non-colored region contains points representing physically impossible cone stimulations.

Visual perception goes way way way beyond just what the eyes' cones can physically see — you've got an entire brain back there trying to interpret optical nerve phenomena!

For a nice brain tickler, look up physically impossible to conically detect Chimerical Colors (e.g. stygian blue; self-luminous red; hyperbolic orange), which can be seen without actually having been seen †

†: https://www.perplexity.ai/search/please-provide-chimerical-c...

W: https://en.wikipedia.org/wiki/Impossible_color