@derekmetaltron put it nicely.
Heart’s Choice games are focused on romance, every other genre comes in second place.
Most Choice of Games are usually focused on genres other than romance, with romance usually coming in at second or third or fourth or no major place.
Where part of the content of a HC and CoG game has to fit into whatever the requirements and guidelines of the companies are, Hosted Games are a wild bunch. Their content is completely up to the respective author.
But for all of them it comes down to the same thing:
If the author wants explicit content in their game, they will probably find a way to add that. I am aware of one upcoming game that is going to have an explicit scene with one of the ROs, the series is likely going to have more of that with the other ROs later on as well. The author is someone who enjoys to write that type of content. I know another author who publishes explicit content for their Hosted Games outside of the forum, and who has an explicit/non-explicit switch for the CYOA game they are publishing with a different engine. This author, too, enjoys this type of content, but maybe it didn’t quite fit in with the story published on HG or they were unsure if it would go through the App Store’s content checks.
Other authors might not want this type of content in their games. There might be warm, loving scenes, describing some action but eventually fading to black. Others will brush over it with a sentence or two. That can be disappointing for some readers, but in the end it comes down to 1. what the author wants in their game, 2. if the author feels comfortable to write that content, 3. if they wouldn’t mind that content in their game but don’t want to write it, do they feel comfortable to have someone else write this content, mess with their ‘baby’?
As @poison_mara said, most of the games that do not have explicit content have a different focus. The authors might feel like there’s little point for explicit sexual content in their games, they don’t see the need for it, there can be a myriad of reasons. It’s their vision of their story. Having someone else tinker with it to add content they don’t deem necessary is probably not something they strive for. If authors do want to add certain content but are unsure if they portrayed it right, they are likely going to ask for assistance on their own volition.
You also can’t simply move a Hosted Game over to Heart’s Choice. Heart’s Choice functions in a similar way to Choice of Games, in that they sign a contract with authors to write a game for this label. Hosted Games are all independent of that. They do not have a contract with the HC or CoG company. A game released on the Heart’s Choice label would also need to fit to the company’s criteria, which is not always the case with Hosted Games. As it’s one of the company’s own labels, they’d also need to do a lot more editing and proofing work, as they have to make sure the game meets their standards, which I imagine is already a daunting task with the games they license themselves. Adding a bunch of previous Hosted Games to that would do the staff little favor.
There’s also costs involved for hosting/publishing additional versions of these games, and further workload to update these games when bug reports come in.
And in the end it’s like this: A Hosted Game does not have to be PG. A Heart’s Choice game does not have to be Mature. Hosted Game authors can do whatever they want, and they likely do whatever they want as long as it goes through some content checks. If their games have that type of content, they wanted it. If it does not have that type of content, they probably did not want it (or didn’t dare yet). One can dream about that content, and it’s easiest to let your own imagination run wild in these cases.
Is it an actual company rule / guideline or more of an unspoken thing?
I’ve been on a kick of playing VNs that are ‘romance’ and… well, dark is a good word, though that doesn’t even touch how dark they are, lol.
