Why is the PayPal pay by credit card option not being displayed?

The guest checkout option will be shown based on your preference whenever possible, however, PayPal will make a decision to not allow guest checkout in some circumstances depending on a variety of details, e.g. customer details, cookies, locale, IP, fraud etc. For this reason, the guest checkout option is never guaranteed for each transaction.

A few of the reasons that the guest checkout option may not be displayed:

• Locale/IP
  The guest checkout option would not be available for all locations, as some countries (including China, and some European countries), are required by PayPal to have a PayPal account before making a transaction. In this same regard, if the IP address of the buyer is determined by the system to be at a location where "PayPal account optional" is not available, they will be required to create a PayPal account.
• Email Address
  If the email address that the buyer is providing is linked to an existing PayPal account, the option to checkout as a guest will not be available since PayPal has already established that the buyer has an existing account.
• Cookies
  PayPal, as a business, does make an effort to create new accounts. Based on cookies within the browser, if PayPal can see that the buyer has used the guest checkout a numerous amount of times, they will remove the option and require the buyer to create a PayPal account. Since this is tracked through cookies, clearing the cache within their browser should restore this option for the buyer.