I'm assuming because ESO Plus is a subscription, meaning it will auto-renew every month without action, and that they expect Xbox credit to not be able to accommodate a recurring payment; either because the system won't allow it to process recurring payments, or because there is unlikely to be enough credit in there.
I had the same issue so I phoned Microsoft. They told me that you cannot use credit on subscriptions.
Apparently because subscriptions are an ongoing thing, you have to have a credit card linked to your account in some way (paypal etc). I said that little piece of information would have been handy to know beforehand.
Credit debit cards don't work either. I have a debit card with the Visa logo but it won't work. It is even the same card I used for my sub on PC account. But it wont work with subs on xbox.
I paid GameStop for the premium deal. Not good. I put in the code and got the credits, could not apply to the premium though. Not happy. I'm assuming it's not doable at the xboxlive store either, huh? Nice.