I wonder if it differs across Europe. In the UK at least, you are very right. Debit cards run via the visa and mastercard networks and every store that accepts debit cards also accept credit cards.
German banks are stubbornly clinging to their Giro cards, much to the inconvenience of iPhone users. The cards are usually Maestro-compatible, but domestic transactions use the domestic system.
I'm sure the privacy aspect of it must be very tricky; Apple collects a lot of information when using Apple Pay, and I can imagine that German banks might be hesitant to share such granular details.
Here in NL Apple Pay works fine with a lot of banks -- and they're still just Maestro cards.
I suspect that it's related to the high degree of decentralization. Sparkasse and Volksbank are each federations of hundreds of independent local credit unions. You can do business with them if you accept that the standard interchange fees and conditions are set in stone, but if you want preferential treatment, you need to negotiate with every local member bank individually.
I'll go out on a limb and assume that the standard fee structure does not afford wallet manufacturers a per-transaction fee.
