DLC and microtransactions requires servers to be kept, itch.io as far as i know is using pay per bandwidth system. many who come to itch.io are doing it because they hate DLC and/or microtransactions, it will be a feature that most will not use. They money spent on hosting DLC/microtransaction can be used for buying the game normally as it should have been.
if a game needs microtransaction/DLC to be kept alive then that is not how you build games, AC;O is doing this and everyone is mad. the game will die out like every other series. Itch.io is not ubisoft it does not have lots of money to keep everyone's microtransaction for years and years. a good game and a good business doesn't rely on tricking customers.