But there's no reason why other app developers couldn't.
Sure you won't get the core responding to texts/iMessages (it can still read them though) but other than that, everything else Android Wear does you're going to be able to do.
But the main purpose of Android Wear is to deliver notifications to your wrist, in order for you to not need to check your phone as often. Google search wouldn't likely be as useful either.
Overall, the effort that Google would have to put in would not be worthwhile in the end, considering some users are staying on Android just to use Android Wear. However, the largest issue with having Android Wear on iOS is Apple - it is extremely unlikely that Apple would allow it to happen.