Isn't that due to green being the highest energy of light for an LED available at that time?
IIRC, everytime a new material was created to get the increase in band gap for a higher energy photon (i.e. blue or violet), there was a Nobel prize given out.
Tuning the band gap with a new material back then was difficult I think.
green has a lower band gap relative to blue/white, ~2V depending on chemistry, but there's something like that going on - IIRC human eyes are most sensitive to green around the ~500/550nm wavelength that green LEDs emit, so you can get good brightness out of low power.
Tuning the band gap with a new material back then was difficult I think.