Jackson The reasoning for this is that with card payments, bunq earns a bit of money through the interchange fee. It's very low in the EU, my personal estimate would be less than 0.1% of the payment amount going to bunq but this is of course not known to anybody outside bunq (like me). In any case that's why they can donate more trees when you spend more with your cards.
For other payment methods like bank transfers, direct debits or iDEAL payments, bunq actually loses money the more they are used. So for them it would be a bit counter-effective to encourage people to use them more. The money goes to the clearing houses and central banks in that case. Maybe they can plant some trees? 😉