I think there are as many as four reasons for the slow progress:
1. A second baby arriving. Looking after two babies must eat up a lot of time, and exhaustion from interrupted sleep at night can't help.
2. From their posts on their Patreon, they are clearly perfectionists.
3. Chestnut produces all the art from scratch, rather than use an art package. This does mean that it's much higher quality, the outstanding feature of the game IMO.
4. There's just the two of them. As a married couple, I can see why they don't want to bring in anyone else, as it would greatly change their work dynamic.