My guess is the means of payment.

With Android and i-products, the payment is directly linked with an account with a payment system where it is practically impossible to hack.

With PC players, there is no such monopoly payment system in place (it is still hackable with corrupted webpage or software), so it is possible that some game developers rather not take the risk of being complained or sued because of 3rd party issues when it is close to 100% safe with the former 2 median (it will be on global A1 news if Apple payment systems got hacked). If that makes sense.