There are fewer blue cones in the fovea centralis than there are in the surrounding parts of the macula, so humans can't resolve details as well in blue light.
The sensitivity of S cones is also simply much lower than that of the M cones. It's clear that pure (0, 0, 1) blue is perceived as vastly darker than pure (0, 1, 0) green. Blue light must be about 10x brighter (in linear intensity) than green light to be perceived as equally bright; the brightest full-saturation blue in sRGB looks about as bright as the very dark green (0, 0.1, 0). The contrast on black background is incredibly poor.
Which is why people who understand color tend to add a bit of green in to make a color which still looks deep blue but is much brighter than what #00f looks like
