Process of elimination:

Black - all shadows and anyone with dark hair is not transparent.

White - really pale people, your eyes, apper are all now transparent.

Red - If you blush or are a bit sunburned, you're now semi-transparent.

Yellow/Orange/Purple - all have a ton of different shades so are harder to key on (with older set ups).

Blue and Green are pretty commonly used. Blues though are pretty common in both genders for clothing - though some news sets will have more than one screen to allow the presenter a choice in clothing.

Green also gets bonus points for reflecting light really well from artificial light sources.

Now - you might point out that we have really powerful computers and lights you can set to any frequency you want now. So - honestly - we could use ANY background we want as a "green screen". And, we totally can. Heck, programs like Zoom can identify things in the background and just get rid of them without even taking color into account. However, we continue to use green because it's just what we've always used and technicians, wardrobe and presenters are used to the quirks, so it's more common.