My guess as to why the saves don't convert is the fact that the devs are using their own proprietary engine. This isn't Ren'py or RPG Maker, Brakus made the engine from scratch.
Given that, there isn't anything stopping him from updating the engine here and there as they create each new update. They've posted things in the past about how they were tweaking the lights to be better, and honestly if it were me, I'd be iterating on it if I had the opportunity, because why not make it better?
With each tweak and change, each update uses an engine that likely has a lot of differences from the last version. Probably not big differences all the time, but little ones all over the place. If you tried to use a save from a previous build, it would break the game too much. Developing a fix to convert saves to the new release would be a lot of work, and probably isn't worth the time and energy (releases would probably take much longer) for the devs.
Not saying I'm defending this approach, just guessing as to why this happens.