Each LED technology has an optimal wavelength. The current tendency is to use blue LEDs and then make white light with phosphors so the development is mostly on

Each LED technology has an optimal wavelength. The current tendency is to use blue LEDs and then make white light with phosphors so the development is mostly on the blue LED technologies. The green problem is caused by the lack of a technology. So you end up with bluish/green coming down from the blue side and yellowish/green when going up from the red side. That prevents really efficient RGB LEDs and makes the inefficient phosphor based white LEDs competitive. If you ever see a RGB LCD backlight you will see that there are more green LEDs than red or blue LEDs.