Surprise….they only accept German issued credit cards.
...and Euros (your "...it’s a reminder to always carry cash" comment)???
What is a German-issued CC vs a French, UK, or US issued CC?
Is it a different network the restaurant is using to validate and process the transaction? Is there a cost savings for the restaurant to opt for only some small subset of credit cards? Would a US issued DEBIT card be valid for them?
Wiesbaden is probably just far enough away from Belgium and France to not really get a lot of non-local/regional visitors, but it does seem like it would be a bit odd to exclude those folks. Maybe it's clear on the website that they are making an effort to just accept local CCs or cash?
But, yes, while I am in the "use a CC whenever possible/reasonable" crowd, I generally always have cash on hand - in the US and abroad. Cash remains a good back-up plan for many situations. Cash can also be a VERY good budgeting tool for folks who need that sort of extra support.