Pay for iCloud with Apple Pay
The real answer is because Apple Pay stores the card info locally on your phone/watch. That's why when you get a new device, you need to re-sync and re-setup all your cards.

In contrast, iCloud subscription is billed directly from Apple's servers with whatever payment method they have on file. They don't have access to the card in your Apple Pay which is stored securely on your individual devices.

In the article about Comcast posted by oa- in the other answer, the actual thing Comcast is allowing users to do is to open their app and make a bill payment from the phone using the Apple Pay card stored on that phone. It still doesn't allow reoccurring automatic payments, because it can't.