Why is my Apple Pay card locked?

I cannot tell you why it is locked but if you follow these instructions you will find out -

• If your Apple Cash is locked or restricted you must go here to start the lengthy process of getting it unlocked or the short process to get it unrestricted - If your Apple Cash account is restricted or locked - Apple Support

Go to your Apple Cash card

• iPhone: Open the Wallet app, then tap your Apple Cash card.
• iPad: Open the Settings app, tap Wallet & Apple Pay, then tap your Apple Cash card.
• Apple Watch: Open the Apple Watch app on your iPhone, tap Wallet & Apple Pay, then tap your Apple Cash card.

You'll see the option to Verify Identity or Contact Apple Support. Tap one, then follow the onscreen steps. Learn more about identity verification. 

1. If your account is restricted, you can still transfer your Apple Cash balance to your bank or use it to make purchases in stores, within apps, and on the web.1
2. If your account is locked, you can't use Apple Cash to make purchases; add, send and receive money; or transfer the money in your account.