Abandonware can a game get removed cause of it?
Of course. To whom would the store front pay the revenue?
doesn't the payment stuff apply to both platforms?
No. Steam is multinational. They could just transfer money internally and pay money to your bank account. I do not know what they actually offer, but they accept a lot of payment methods. It stands to reason they they also offer more than a few methods of getting payout. Itch on the other hand does not want to act as merchant in some situations or the developer configures it such. This is called direct payment. Ever saw projects where you can pay in Yen or Australian Dollar or Euro? Those are direct payments.
Also, if you pay a game on itch, taxed are handled ... not very well. Prices on itch are vat exclusive. Prices on steam are vat inclusive where it is local law, because they are multinational and actually do offer the items in the local markets. And they are converted to local currency in the shop, by the shop.
But in the end, each removed game has its own story, why it was removed. Some stories might of course be similar.