Not all debit cards use the same technology.
A regular Visa card can be credit or debit card. But there's also V-Pay and Maestro. And German banks push their own system (which consequently is the most common in Germany) called Girocard (or specifically Girogo for contactless)
iOS only supports EMV credit cards (even if they really are debit cards). iOS 12 will add support for V-Pay, Maestro and Visa Electron.
Support for Girocard is rumored to be a negotiating point between Apple and the Banks, as are distribution of fees.