Because it is.
ARE YOU RACIST? What's wrong with yellow skin huh?
Anyways, before someone reports me for "falsely accusing someone of racism who may or may not be racist...which I think is also not reportable either", I looked online for you:
"Bananas are yellow in colour due to the effect of the artificial ripening process. Bananas are typically stored at about 17 degrees Celsius and then treated with a low concentration of ethylene. Bananas which are allowed to ripen naturally on the plant usually have a more greenish-yellow form which then changes to a brownish-yellow as they continue to ripe."
