The way I see it, the dev probably never expected this much success from the game and thus listed it on itch. Then it exploded and a steam release made sense. Now that release has features that are, presumably, explicitly supported by the steam API (read: not itch); imo enough new features to justify another pricetag.
That's a lie though actually. The way I really see it is that I could pay for this game in fucking pennies, get over yourself.