I briefly went over their guidelines and saw they demands $100 upfront just for setting up a shop, but is that the main reason for not selling there?
I'm just making an educated guess, but I wouldn't be surprised if this was a recent change intended to slow or hinder the proliferation of the aforementioned extremely low-effort, cheap puzzle and asset flip games. Steam has grown by leaps and bounds since it started and gotten so big... I wonder if they can still afford to have an actual person manually review all applications for setting up shop or if they use algorithms for filtering at least some of it. If it requires $100 to set up shop, that might get some fly-by-night types to think twice. Otherwise, what is stopping a cheesy asset flip game developer from just changing their name and IP address (and contact info) after getting banned by Steam and trying again?
To add weight to my guess: A quick Google search finds articles discussing new changes by Steam, with the headline mentioning $100 for opening a shop. These articles are dated June of 2017. As I recall, I think 2016-17 was around the time Steam was getting unwanted attention for having so many cheaply made games.
IMO steam was very picky with porn content in the past...
They still are. But they have to be. There many potential legal complications that the average person or gamer is unaware of when it comes to rendered or cartoon porn or erotic games or art. And at least some of these laws seem contradictory or do not make much logical sense.
There are certain nations - Australia, Canada and the UK in particular - which have very strict laws about art and cartoon characters that are portrayed in an erotic way. Potentially, these can involve hentai, erotic manga and even personally hand-drawn art. Believe it or not, there are even some similar laws in the US, though not as strict and very rarely enforced (and, perhaps, very open to a judge's interpretation).
For example, people have been arrested and even jailed for crossing into Canada with certain kinds of manga in their possession. Recently, Australia has clamped down on not just obviously erotic anime and manga, but even some very popular anime that you would be surprised - titles like No Game, No Life, Sword Art Online and Goblin Slayer. From what I've read, Australia can charge someone for possessing cartoon porn with, for example, the likeness of The Simpsons characters. And in the UK, at least from what I've read, it sounds like a judge can rule that a character is "under-aged" merely because the character is flat-chested, regardless of any other factor. And such rules also apply to games.
Despite what some believe, having a disclaimer of "all characters are 18 years of age or older" or similar does not remove liability. Legal trouble can still occur. Generally speaking, it sounds like courts rule based on how characters look while often ignoring disclaimers or the character's stated age. For better or worse, it seems having "anime-style" characters is what is most likely to get someone it trouble. (This is not legal advice and I'm not an expert. Obviously, anyone developing or distributing an adult game intended to be sold to the public should seek professional legal advice.)
Anyway, Steam and other online marketplaces have to be very careful because they could, potentially, be held liable for "distributing" illegal content. Steam does allow some adult games, including some anime-style. But don't be surprised if Steam suddenly removes an adult game without warning or explanation. I've seen it happen several times. Sometimes the removal is understandable, involving stuff so questionable that one wonders who was so inept or half-asleep that greenlit it in the first place. But sometimes the reasoning is far less obvious. Heck, I wouldn't be surprised if some of the ones that get pulled got 'culture canceled'.
Granted, there are some small, rather obscure online game stores that are willing to host and sell adult-oriented games that, for example, got rejected by or pulled down off Steam. But some businesses are also more willing to take risks.
