My close associations in Germany indicate MWST is the driver for that, just as elsewhere when the taxes are extreme. Cash plus spoilage makes the world go around.
MwSt is just VAT and I very much disagree. Only once, I did not get a receipt for my landing fee and that was a very small grass airstrip operated by a club where the statement was "there is no landing fee but we wouldn't object to a small donation for our club".
Also, I have never paid for fuel in cash, all aerodromes I've come across offer payment via credit card or at least Maestro/EC. Some of them charge a fee for credit card payment. Maestro cards are very widespread in Europe. If you plan your fuel stops at public airfields (Verkehrslandeplatz), you can expect credit card payment.