I'm going to take a stab at this question for you. It's been answered before, but this time I'm going to add some more context to everything so that hopefully future readers can see why the Devs are working in the manner they are.
Unfortunately for them to add a new girl it costs money because of the amount of art that is needed.
You have a body, a head, and limbs. Then you have their brows, eyes, and mouth. Add on to that any garment (each of which has at least two drawings). Currently there are ~250 images that go into making a single girl's standing actions. This is before then including their bedroom background, their bedroom items, and any gifts which are displayable which is an average of ~50 images.
Since they are already expanding on the poses of the existing girls to have more dynamic poses, this count is already going up. But we'll go with 250. Obviously some drawings are more expensive than others. A single set of eyes isn't going to cost the same price as the bedroom background. So we'll just go with an average and assume that the artist is dirt cheap - so the average is only $2.00 per image. (I am 99.99% certain this is way lower than actual.)
300 images @ $2.00 = $600.
And that doesn't include the sex poses. Adding in all of those adds several hundred more images. (Average is ~750). There are some which might be able to be recycled, so we'll be really generous and call it just 300.
So now we're at 600 images @ 2.00 = $1,200.
600 images per girl to add means that the artists are going to be swamped adding new girls. With only 30 days a month to do it, that's 20 images they would have to knock out a day on average just to get one girl done. The artists are good, but that kind of production level is a full time job at nowhere near a full time pay. And that assumes not going back and expanding anything for the existing girls on that time table.
Up until the patreon launch everything had been self-funded. Now that the Patreon is starting to collect money they can create a pipeline to get art done regularly, but that still takes time.
If they try to push the plot ahead too much they are going to need more rooms than we currently have, which means needing new backgrounds at minimum and that puts us back into the Time/Money loop discussed above. If they try to add all the girls art immediately, they again are in the Time/Money loop.
So instead the Devs are balancing what they can and adjusting based on how the Patreon performs. The better that does, the faster they can commission art and do both improvements for the current game as well as order art for future scenes (be it girls, backgrounds, etc).
When they can't work on pushing the story forward, they instead work on addressing things in the existing build. This includes taking the feedback from the players and tweaking things. This is why v0.3 is so focused on some of the system overhauls to help reduce the feelings of grindiness, expanding on some of the existing expressions, and otherwise working to fix more of the little things that didn't quite land right.
Hopefully this explanation helps better illustrate why the Dev team would be working in the manner they are as they try to budget their resources of time and money to produce the best quality product that they can.
