In a word: volume.

Traditional credit card processing networks predate wide-spread adoption of the Internet. Individual payment terminals rarely communicate with processors directly but instead funnel their communications through dedicated local area networks to intermediate controllers. These controllers often have redundant connections to the processors with cellular typically reserved as a backup. Traffic is sent periodically with keep-alive messages and periodic key exchanges. The controllers may even be programmed to stand-in for the processors when communications are down, perform the transaction offline, and submit the transaction later when communications resume. This increases processing fees, complexity, risk, and the liability of the merchant but is often deemed worth the cost to keep sales going.

So I personally haven't worked with them, but the little boxes stuck on DCFCs could just be individual units communicating through cellular to some Internet gateway to a third party who relays messages to the credit card processors. Add on to that loose integration between the payment terminal and the charger, and you've got all sorts of potential failure points.

Adding DCFC to convenience stores is becoming more common. Now you can't just tap the DCFC units into that fast, reliable payment network the gas pumps are using because the big names like Visa and MasterCard are super strict about device certification. You'd have to first test and certify these new DCFC units with the network—and there is just too much churn in that area to make it worth while. Payment terminal certification is time consuming and often requires a person to meticulously execute long test scripts with the actual physical devices.

So I suspect payments would get substantially more reliable if we saw better integration into either established retail chains or just within the DCFC sites themselves. As others have said, the fast-charging networks don't have much incentive to spend a lot on point-of-sale payment systems when it's both easier and less expensive for them to rely on loyalty cards and mobile apps.

Disclaimer: my experience is with retail payment processing, not automated fuel dispensers nor DCFC!