You're paying for a game that doesn't have the same features (in this case, multiplayer, which is a big part of the latest update) as the Steam version. I've never seen that anywhere else.
The game is literally the same price, or even a little cheaper on Steam with the current promotion, so what are you talking about? And even then, we're paying for a game to have the same one everywhere, aren't we? I'm not spitting on the development, but I think it's a bit lame to force yourself to release an update just on Steam, ignoring the Itch'io buyers who contributed to this development.