I bought a new iPhone 12 in October. Why am I not eligible for the free trial of Apple TV ?

You should be, but the window is about to end or already has. You have 3 months from purchase to redeem the offer. If you have not redeemed it yet, it may already have passed depending on when ion October you bought the iPhone.

See here: https://offers.appletvapp.apple

Otherwise, you will need to contact Apple support directly: click here: Get Support

I suggest that you contact Apple directly with this question as this particular offer may have changed, was not available, or the country you reside in is not eligible. No one here knows the "why." Only Apple would. Good luck!