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.
I'll go out on a limb and assume that the standard fee structure does not afford wallet manufacturers a per-transaction fee.