Why are the three component LEDs in an RGB LED so unbalanced?
Sounds about right. To get white (6500K) using NTSC (colour TV) phosphors, the relative intensities are G=0.59, R=0.3, B=0.11 - most of the energy is in the green, least in the blue. (slightly differently rounded numbers in Wikipedia ) At equal intensity, blue would appear brightest. The actual numbers will differ here (LEDs not phosphors) but the relative intensities are actually more similar than I expected.

Spehro's interesting comment goes some way to explaining why. The Candela is a definition of luminous intensity that is weighted such that 100mcd of red, green, or blue light are perceived as equally bright.

Now as I understand the colour space conversion process - it doesn't follow from that, that mixing equal perceived intensities of R, G, B will result in what we see as white! 

Indeed how can it? Our eyes are most sensitive to green. So the actual intensity of the green light is reduced in the definition of the Candela to give the same perceived intensity as red, blue (Nitpick : I believe the other intensities are increased instead). Then, to mix the three and make white, we need to increase the perceived intensity of green light to restore the correct intensity in the mixed light. (That is why the measured intensity must be greatest at the wavelength where our eyes are most sensitive. That wouldn't make sense otherwise!)

In other words, 100mcd each of red, green and blue contains much less actual energy on the green channel, whereas true white light would contain approximately equal energy in each channel - hence the definition of "white noise" in electronics.

EDIT : An interesting article places the quantum efficiencies of red and blue LEDs in the 70-80% region, well above that of (previous to 2008) green LEDs (it's a sales pitch, after all!). This makes it likely that, whatever the reason for the low intensity of the blue LEDs, it isn't that they are difficult to make.

So the relative intensities of the three LEDs in the question is the manufacturer's attempt to undo this weighting and match the LEDs so that the light generated is approximately white at rated current. 

Illustration (image source)

To my eyes at least, in the illustration above, G is by far the brightest primary, with R second and B darkest, yet when mixed, they produce a pretty good white.