Unfortunately, I have no idea... I don't really use the Itch app myself. But I'll try to look around and see if I can find a reason.
Was just curious since the Demigod Trainer Christmas is downloadable in the app, but this one isn't. Great game though, can't wait for more content :P
I just learned about this today for my own game, I think you have to flag the game by the platform for it to be findable in the app. good luck!