I would like to apologise for my previous comment. I meant it in all good faith and curiosity but reading it back it sounds more to me like it might come across less so and more entitled.
I have since googled this question on my own (why cool game only on steam :c) and have learned that there are a number of financial and labour related reasons why it is less difficult and more rewarding to only release games on steam rather than on steam and itch and gog and wherever else. I feel saddened by this but ah well.
Having said all that, I have now played this demo. It is gorgeous and wonderful and deeply unique. I would very gladly buy the full version if it ever were available off steam, but otherwise thank you for such a fantastic game.