Hiya! Thanks for your message. :3
The Patreon offers a Steam key as a reward so that long-term patrons can play an up-to-date copy of the game, even when they’re no longer subscribed.
Since the itch.io version is a standalone version (that also receives updates), there is no need to provide a Steam key.
For those that prefer Steam DRM, we will start selling on Steam in February. Hope that makes sense!