I mean, I am a scientist and I don't know. ¯_(ツ)_/¯ But like you said, maybe it has to do with the goggles being sensitive to infrared and red is close enough in wavelength to infrared that it can overload the gogs.

As long as we're speculating - here's a substantially unsubstantiated web page about it. It says green light at low levels also doesn't mess up night vision. And green can be used at lower brightness, has better contrast and color discrimination, and is better for common tasks like reading maps.