There's a full answer to this question in the FAQ, but to summarize it: Steam is so flooded these days that I think it wouldn't be worth the effort to make a release there for me right now (there's a $100 fee and a lot of paperwork involved in getting a game on there). If I ever get a reason to change my mind (e.g. I get a big enough following that I believe people will actually find the game, or Steam starts actually having quality control again so it's easier to get noticed), I will mail a Steam code to everyone that bought the itchio version (and set the game up for the cross-platform purchase thing for future purchases as well).