If you think about the social security number system, paper checks or credit cards with magnet strips I think you'll notice that other countries sometimes have stricter and more advanced security regulations.

Or maybe even different companies forcing users to accept credit cards in different ways.

A handful providers I had to integrate with in my career (in LatAm and Germany) had this rule where you couldn't have the numbers going trough your system unless you got PCI certification. You had to use an iFrame, or redirecting to their website where the form was served.

Sometimes the APIs were there, in public, but even if you used a valid credit card number it would deny verification unless your merchant account was pre-authorised.