the creator does not want to put it on steam because there is a lot more pressure putting things on steam
Steam takes a BIG cut and doesn't permit upcharging. It is a big expense for an indie developer who has to determine if the increased exposure on Steam will cover the added costs. Apparently these developers have determined that it will not. I guess they are in that sweet spot between being small enough to not afford the cost, and big enough to not need the exposure. So platforms like Humble, Itch.io, and direct make more economic sense to them.