Family sharing - why is only the organizer's credit card able to be used for subsequent purchases?

You can still use Family Sharing and turn off Purchase Sharing to allow each family member to use their own payment method. You can still share subscriptions with purchase sharing turned off, just not apps or books. Also if you want to leave purchase sharing turned on, each family member can redeem gift cards to their Apple account, then any purchase will come from there first before being charged to the Organizers credit card.

No Problem. The organizer will need to be the one to stop Purchase Sharing so each family member can use their own payment method. As I said the only drawback is not being able to share apps purchased from the App Store or Books that were purchased, but in most cases that is not a deal breaker. That way each person is responsible for their own purchases using their own payment method.