Because banana's turn from green to yello as they ripen. This in many South African tribal dialects is called the "ban-yah". The word was brought back by Spanish explorers to mainland Europe and then modified to fit the English tongue.
-spence
OK, OK, if they call an orange an orange, right, why don't they call a banana a green to yellow????????????????????? I'm not going anywhere near the "modified to fit the English tongue" stuff