I'm also looking for reasons behind things all the time and i also felt the current endings to be a bit abrupt as you said. So i thought about it after they were released and this is what i came up with.
First, the obvious one. There was a time jump. This might be the first one in the story, since everything else before the ending happened within the span of a week or two and the endings are several months later.
Second, my main route was the harem one and it's ending ending is yet to be completed. I checked out the other routes too, but mostly just out of curiosity and to see how well they are handled (very well, in my opinion). So not only were the endings a time jump away for me, i also had to jump to a different story line.
And finally, because this is a short story and if TrustyOldPatches could magically create 10 more chapters overnight, i would gladly read those too. This story was always going to end sooner than i wanted it to.
Your reasons might be different, these were mines. So, for now i'm just waiting for the main ending and the new game. In the meanwhile i grin a bit each time i check back here, because according to the last public sneak peak on the patreon, i got the harem picture right 4 pages ago.
