I don't know for sure, but here's a guess:
To get a credit card, you have to pass some basic checks: your age, your income, your credit history, your ability to repay. If you have a credit card, there's a much better chance that you do have the ability to pay for whatever you may order - or at the very least, Paypal takes the money from your card and your card provider takes the money from your bank account / hide.
Signing up for a Paypal account is relatively easy. Even if you have a Paypal balance, that balance may or may not be secured. Sellers may be taking additional steps to protect themselves by requiring that your Paypal account have a credit card attached to that Paypal account.
The above is all pure speculation on my part. The logic seems solid, though.