To be honest I don’t know myself, as with many of our preferences I don’t think there’s necessarily a good reason.
But if I had to give a reason, it’s probably because it’s between wifi and Wi-Fi. Using Wi-Fi isn’t great on phones or keyboards because you need to add the “-”, which isn’t typical in most typing. But then, just using wifi doesn’t feel right because it’s too far away from Wi-Fi. So WiFi falls in between.