The B&W switch on the 2600 sets a register that can be read at runtime allowing for color changes that better support black and white TVs. Howard Scott Warshaw famously had only 5 weeks to complete the entire game, so supporting that feature probably didn't make the cut.

NTSC color artifacts may be a factor too, but not sure. I know green and red aren't too happy next to each other. Making ET an actual brown shade may have gotten too close to that red-green conflict.

This reminds me of those bug reports that say "Button is green" and you're not sure if "Button is green" is the desired or undesired behavior.