Apple loves to make their devices as thin and light as possible. That's why they maintain that status quo: they make their batteries only as big as they need to be to last a day. If this technology improved capacity by 50%, that means Apple could shrink their batteries by a third while still hitting that target, which they'd jump on.
(Personally I really wish they'd have a model with a much larger battery, because "lasts one day" really means "usually lasts one day but it depends on how you use it so you can't really count on it." But they won't.)
Justification: Apple would very much like to avoid a Samsung-level PR catastrophe. That's a pretty good reason to avoid pushing the envelope right there. They also have their battery life tuned to where they like it wrt the rest of the internals, but if you get a free upgrade in capacity, I doubt there's too much more they could do with the extra space. You're right that they would probably try to figure something out though.
They also need high volume guaranteed before they can put any part in an iPhone and they probably want it to be tested enough to not explode in the user pocket.
It's challenging to achieve for a new part. It's like there is money for much better and much more expensive... but it doesn't come for free.
Apple seems content to merely maintain the status quo of barely a days battery life. I would not expect them to lead the way on this front.