Why I couldn't get one year free subscription on my Xmas?

If you’ve already activated the 7 day trial you likely can’t change to one year now.

Offer must be claimed within 90 days from when you activated the new device 

Open the Apple TV app. Make sure your device is running the latest version of iOS, iPadOS, tvOS, or macOS.

The offer should be presented immediately after launching the app. If not, go to the Apple TV+ channel page where you’ll see the offer.

Tap Enjoy 1 Year Free.

Apple TV + Offer

https://support.apple.com/en-us/HT210573

Was the iPhone purchased recently? If it was done previously then the offer would have only been valid for 90 days.

Was this your first purchase of hardware from Apple since September? The offer is only valid on the first qualifying purchase.

If yes to both and you are still not able to claim then best to contact Apple.

It is well documented that the offer is only valid for 90 days, and users are also notified when they are approaching that expiration. Once it is passed it will expire. Of course one can contact Apple to see about an exception, however, is would be within their discretion and most are not.