http://i.stack.imgur.com/6kuZf.png

Blue LEDs are much more efficient than most other colours and having three sets of voltage control or three seperate circuits is more expensive. Plus "single wavelength" is only even approximately true of a laser. Those spectra you linked to are still a fairly broad range (to the point where red and green can even be somewhat distinct under an orange LED even if both are very orange-ish).

Additionally your source describes a modern white LED

> The other method of creating white light is known as spectrum adjustment.

> Spectrum adjustment happens when light is absorbed and then re-emitted again. By doing so, the color and appearance of the light can be altered. There is a caveat, however, that the light can only be shifted to larger wavelengths. If a material were able to absorb a long wavelength of light and emit the same amount of a shorter wavelength, this would violate the conservation of energy. This is why it was not possible to generate blue light from other diodes. However, the reverse process does in fact work: if a blue diode is passed thorough a yellow phosphor, the blue and yellow combine into a white light well suited for everyday tasks.

RGB LEDs are at least three (and sometimes four) LEDs though.