If I had to guess why BoN is so slow nowadays, I would say:
First, there is no competition. This dude is the only one using these models on tk17 animated with this amount of content. The demand is too high and the supply is literally only him. He has no incentive to keep working besides his work ethics (which are questionable recently).
Second, he keeps doing everything alone even after all this time. On the beginning of the development he was dropping so many updates so quickly, it was one of the best """regularly""" updated games at the time, thats why the old version was stacked with content and a lot of the revamp just uses them. He clearly has slowed down now, but still doesnt want help.
Look at eroniverse with confined with goddesses, this guy was doing hand drawn unique artstyle with monthly updates consistently, and guess what, he understood that it would be too demanding. He was transparent, changed to another artstyle which he enjoyed AND was easier to pump out content and STILL, this week, he announced he might starting delegating the work so he doesnt have to do everything alone. That is absolutely amazing, he knows his limits and its going to fuck up his update schedule if he doesnt get help, he made the right choice.
Why cant beggar do this? No clue. If only we had a debugger before release, the updates already would be a lot more stable on release. A writer to help him, something! He just has no real incentive to deliver great content, why work hard if you dont have to? The paycheck is coming anyways. That just comes down to each developer morals again.
Wow... I cant believe I became a wall of text andy for this game... Guess I just really like it.