My guess? Most green things you see are a darker green. The extremely bright green (lime according to the W3C) is less useful, so they probably chose the color based on what they thought the user would expect.
Given that RGB is well-known, I suspect their assumption was wrong, but I have nothing to back that up.
Also, green is the brightest primary after accounting for typical human vision and typical monitors. The YUV formulas put at nearly twice as bright as red.
So a normal display has more green than you'd need, and 00ff00 green has terrible contrast against ffffff white
Extra nitpick.. green isn't even a primary colour. Red, Yellow, Blue are the primary colours that our eyes perceive. Perhaps this contributes to why the RGB numbering doesn't meet human perception of green?
Those are primary pigment colors (arguably... Should be magenta, cyan, and yellow). Those remove light to make color. Red, green, and blue are the additive primary colors.
No you have S, M and L cones in your eyes, which represent (roughly) blue, green and red.
Your eyes are most sensitive to green and yellow because the spectrum of the M and L cones overlaps. This is why a lot of sRGB is green and why the Bayer pattern has one red, one blue and two green elements.
Red, yellow and blue are the primary colors used in art education.
Given that RGB is well-known, I suspect their assumption was wrong, but I have nothing to back that up.