- it's more difficult to implement than FB sign in and websites don't get any of the benefits
- it works with apple devices, and it will be a nightmare to use in other devices, esp. if the user uses the private email
- Apple would control and route the emails of the user and the app! This is a big privacy hole imho.
- Apple drops users or apps without second thoughts from their ecosystem, and the web is not used to this (and is not going to change)
- Maintaining new sign options is already a hassle, adding a new one that will only be accessible to a relatively small user base, with a number of hoops makes it seem less worth it.
On the plus side though, you do know that users coming from apple sign in are probably richer and better monetizable.