it's always the business schedule. If he didn't have to upload anything for monthly subscribers then he wouldn't have to make these short stories. In a way he has backup plans (as he mentioned early he had a lot of new ideas and wrote it down to do it later) just to compensate for months where he couldn't finish the game event in time.
Developing a VN/game where you have to push out a new scene (with CGs, standing art, animation, video) every month is just asking for stress. Normally there are many artists who hold up until near Comiket to finish their doujin too, but that's once every 6 months or once a year. He gimped himself with once a month. This subscriber plan works better when you draw 1-4 pages on twitter/pixiv and provide different CG/extra pages in paid plan.