Of course, may not want to develop for Apple for other reasons (store policies, not wanting to get randomly rejected, etc), but I'd assume money is the driving factor for many.
Now, that isn't to say that Android won't be making good profits. It's just that Apple has a cash cow with their App Store's 30% cut (and now mandatory payments in app). Actual market share matters less, because that's not necessarily where they're continuing to make money.