It's probably just aesthetics. Those colors are more commonly used in illustration and design, so they tend to get labeled. There might be some perception involved in there as well as it's easier for our eyes to pick apart the more pastel colors from each other than the darker colors from each other.

i would expect the more dense part to be the smaller gamut that can be made with paint since we've been naming those colors for a lot longer than the larger gamut that can be made with a screen. The paint/print gamut looks kinda like the more dense parts of these scatter plots within the larger sRGB cube (though the paint gamut isn't entirely contained within sRGB).