Why do they give you underarm rather than forearm crutches for injuries?

I have a weak ankle that is prone to injury so I've spent several periods of anywhere from a few weeks to three months on crutches. Hospitals and doctors always give people with ankle injuries underarm crutches but I don’t understand why. I personally find them almost impossible to use as they are painful and I can barley lift my weight on them. I always end up having to go and hire forearm crutches. Why is it that underarm ones are standard?