When they really need to. But they'll fight it hard! Apple's business model is to lock everything down -- very closed indeed. This does not align with the goals of bitcoin, at least, how I perceive them.
tl;dr Apple = closed. BTC = open.
Apple is a very big and rich company. So it is influential. When they accept bitcoin then it crypto currency will become more popular. It will be so good to hear that apple supports bitcoin.