Art is the primary reason. I didn't have art for it commissioned yet and I wasn't going to do a second scene in a story day where it's two characters having sex through sprites. The monetary costs of those CGs aren't even too much of a concern, but they take a lot of my time to make the design docs for, it takes an artist a few months to get the scene done, and then I have to edit the files to work within Ren'Py. If I stop work for art, this visual novel will not only never get done, but those scenes with the art aren't going to be nearly as good as they could be if I spent more time writing the characters to have a better idea of who they are and how they work so that when they have sex it is an extension of their dynamic rather than erotic revelry for the sake of erotic revelry. My priority is always going to be developing the story and its characters, and that specific scene is not at all a load-bearing scene to that story. If it's going to exist, it'll be as something that moves the plot forward, and there are just other scenes that take higher priority of the bottlenecks of art and my own limited time on this earth.
The other less important reason is that development is slower than in other similar games because of my attention to detail in development and that I am essentially a solo developer (see this previous comment I made for the specifics). If I limited updates to a full story day each time, this game's updates would come out spaced several months apart even in the best of circumstances because writing is merely a fraction of my job, and I'd rather release the game in chunks more frequently so I can get more immediate feedback on what did or didn't work as that has been instrumental in making me a better developer and more specifically a better writer. If I don't have confidence in an idea, I have the luxury of being able to move ahead in the story and to put off the specifics until I have a better idea of what I want that placeholder scene to be. This not only speeds up development by letting me circumvent a plot idea until I have full confidence in my ability to execute it as well as I can so I can do something I do have a much clearer picture for, but it improves the overall quality of the game by removing the pressure of the bottleneck of having to do the next chronological event. If I have learned any lesson during development, it's that it's not just faster but better for overall quality to make sure I do something right the first time instead of going back to fix it later.