Why is my credit card denied?

Nobody here knows. We're other users, like you. Not folks that work for Apple.

Use the Get Support link in the upper right, and discuss this with the folks that do work for Apple.

See the Apple Support article(s) below. Link to contact Apple Support in the article. 

Apple ID payment information - If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store.    

Possible fixes.   Fix ‘Your Payment Method Was Declined’ Error In App Store

                          Payment method declined in iTunes or the App Store.