Implementing a mandatory payment method was done to enable buyers to be able to purchase from multiple merchants in a single transaction, yet still enable automatic split payments between the marketplace admin and the different merchants. This multi-merchant shopping cart experience can only occur if all merchants are on the same payment gateway.
Consider the scenario below:
Buyer X purchases 2 items: one from Merchant A, and another from Merchant B. Without a mandatory payment method, Merchant A opts for PayPal, while Merchant B opts for Stripe as his chosen payment gateway.
Now, when Buyer X wants to checkout with both items in his cart, he is unable to automatically pay both merchants with his PayPal account. Instead, he would be required to checkout each merchant items separately, one merchant at a time, which is an extremely poor user experience.