betplay's devs were tasked with reducing "payment failure" support tickets and realized they can almost guarantee zero payment failures by only allowing one destination and keeping a set of channels with that destination well-stocked with lots of outbound capacity

they have an exclusivity contract with WoS. Payment processors sometimes give merchants benefits if they agree to "only" use them, and if the merchant is popular, this attracts more users to the payment processor's user-facing payment products. E.g. lots of people might get Visa cards if they were the only supported payment method at Disney world, and maybe WoS thinks lots of people will start using their wallet if it's the only supported LN method at betplay. It would not surprise me if LN companies try to arrange such deals at some point