Hi Cobaya (and others who have had this issue),
Apologies for the delay. There seems to be a conflict between Steam's policy around distributing keys and itch.io's policy for supplying Steam keys with purchases. I contacted itch.io support about this a while back but have yet to hear back from them.
Since it may be a while before this issue is resolved, I have manually requested a Steam key for you. You should receive it within a few hours.