Why having a Turkish Apple ID I can’t add my European credit card to the Wallet for Apple Pay usage?

Hi Borman158

Welcome to the Apple (user-to-user) Community.

As we are all Apple users, like yourself, we often do not know why Apple does what it does ....

... sorry to have no explanation to offer.

Apple support have articles online about Apple Pay, and where it can be used:

Eg: Some financial institutions may not participate:

Countries and regions that support Apple Pay – Apple Support

Also some Apple devices may nor be compatible with Apple Pay:

Devices compatible with Apple Pay – Apple Support 

Apple Pay users may need to be signed in (to their device) with their Apple ID.

Sign in with your Apple ID - Apple Support

Hope this is helpful.

All the best :-)