It's not available on Steam because the reviewers at Valve decided it wasn't appropriate for the platform and it was banned. I wasn't given any chance to appeal the ban, and I was told I wasn't allowed to resubmit the VN for approval even if I edited things, so I'll never be able to release it on Steam.
It's unfortunate, but that's just the way it is!
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The decision to ban Chaos;Head was probably reversed because it's a high budget VN made by a well-known company with a big fanbase, so they had enough clout to appeal and get Valve to change their mind.
I am one person with little in the way of financial power or a fanbase, so I very much doubt Valve would ever change their mind re: banning Sweetest Monster Refrain, as they have no incentive to do so.