What could be the reason for that? It makes no sense.
Have you tried GOG?
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Steam is infamous for censoring games that resemble anime and contain more sexual themes. A relatively recent example was the game "Order Us!" which got taken down because they deemed that one of the characters was a minor due to her having a smaller bust size, even though the characters are over 18. If you want to check it out the game for yourself here's a link to the demo with the devlog having the full game links.