It's because they are two different apps on two different phone systems. If you got a new Android phone, you could redownload the app at no charge. But since it's a different phone system with a different app store, there's no way to move it over and you'll have to pay the $3.99 again.
Sorry about that!
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford